"Sealing" or the "sealing power," or "being sealed" is a central part of the Lord's doctrine.
A seal, most recently in history, was made by placing melted wax over the outside of an envelope where the envelope was shut, and then the king's or other royal dignitary's "seal" - a crest, an emblem, a letter, was placed on the wax until the wax cooled, thereby "sealing" it shut.
Nobody but an appointed representative of the king or other such authority could break the seal or open it. If the wax were broken before it reached its intended, designated recipient, the one responsible would be facing a visit with their maker arranged by the king's forces.
However, part of the gold plates of the Book of Mormon were sealed shut with a metal band around it, which at least one account described as having been oxidized copper (green). Copper is a very hard metal compared to gold, and it would have require some ingenuity to make a band that precisely matched the dimensions, was squared up, could hold the pages without damaging them, and as also secure and all done with whatever tools were available while on the run from an enemy that was hunting you to kill you because of the color of your skin.
Of this sealing Moroni wrote: 4 "Therefore I do not write those things which transpired from the days of Adam until that time; but they are had upon the plates; and whoso findeth them, the same will have power that he may get the full account." (Ether 1) And whoever didn't, wouldn't and couldn't.
The plates lay since ~421AD, now it was 1827AD, and in all that time with 1,426 years of civilization all around, a simple rock lay undiscovered with golden plates in a cement box?
They were sealed against man's discovery so that no one but one designated would never find the gold that was laying under their very feet. The work was sealed up by a prophet and the Lord with the earth itself.
Now, this is either a completely fantastic story that happened, or it did not. If it didn't, then where is the truth? And if it did, well then this is yet another witness that this truly is the Lord's work, it's sacred and its protected.
This is a seal to King Tutankhamun's tomb. It was tied with rope binding it shut, but the seal on the right made of clay with a king's seal upon the clay, so you would know if the tomb had been broken open by whether or not the rope had been cut or the seal had been broken. Again, this came with a penalty, it was more a fear of consequences from authority than it was a physical deterrent - a testament to the power of the king, and in the case of King Tut's tomb, after the kingdom head fallen, man was able to break through. But with the Lord's holy scriptures, even the prophet himself was thrown back when he tried to access the records improperly.
But what made the seal of force? The angels, hosts of heaven, and the elements themselves all know who their Master was and they were bound in love and service to obey.
However, seals come in many different types, not just the legal or the physical seals.
Another type of "sealing" - meaning it cannot be opened without the key is language. At the Tower of Babel, the people all spoke one language. They had the ancient mysteries, the holy oracles, the eternal secrets and they all spoke the same language - some studies tell us that they did not speak the same language as Adam, but they did speak the same language, and could trace back through the centuries to the times of Adam according to the records preserved via Noah (I wonder what that record library on the ark looked like!). Lehi took records, obviously Noah would have as well - it would have made no sense to preserve a family by the hands of God but leave their origins behind them. Though we don't have record of this now. But, once we have knowledge we are accountable for it. And the people of the Tower of Babel, having spoken all the same language, having the mysteries, and having one mind were protected by the confounding of the languages so that they could not fulfill their purposes, but also the counfounding of the languages was its own type of scattering. If they could not understand one another, they could not be unified, and would be unable to work together - the notion of translation being a completely foreign concept to them, yet one we work around readily today.
And so with a confounded language, the mysteries of the Ancients were cut off from them, or they from it. It was sealed shut.
So it is with the Book of Ether. The Book of Ether was sealed not just with the bands that Moroni bound it with, but it was also sealed with the word of the Lord (see the First Prophet's words above), but it was also written in a language that nobody could understand except they had the gift of the interpreters which were provided for the translation of the book - only through a prophet of the Lord.
Notice that even though he had the Urim and Thummim, that the Prophet himself even noted, it wasn't until after they had been baptized and received the gift of the Holy Ghost that they began to truly understand what they were reading/translating. The Urim and Thummim or "Seers" did help them translate what had never been there before and make sure that the wording was correct in the translation. However it was the Holy Ghost that unsealed the knowledge* - not just the language.
We read of another type of sealing in the Book of Revelations, of a book that was sealed with seven seals, and that no one could open, none were worthy except the Lamb of God.
A similar sealing takes place during a priesthood blessing, which is finished with the words of "I seal this blessing upon you in the name of Jesus Christ, amen." That blessing can only be broken by one of two ways, by the person's free agency, and by the Lord Himself. But "I, the Lord, am bound when ye do what I say; but when ye do not what I say, ye have no promise" (D&C 82:10).
Similarly, so it is with the sealing of husband and wife, for time and all eternity. They are bound together - sealed - and nobody can undo it except those involved, and the Lord Himself.
But the Sealing Power is closely guarded, only certain men - priesthood holders - can seal a husband and wife together, and that is not at his will, but the will of those involved. A priesthood holder can seal a blessing, a Patriarch can seal a patriarchal blessing. A Sealer can Seal a marriage, but only a prophet, seer and revelatory can break the seal who is not the Lord or the persons involved.
And perhaps the last sealing that comes to mind is that of knowledge. Without knowledge of the Lord, you cannot comprehend the things of God, they are sealed away from you, and sealed up, hidden up, never to be understood - the darkness comprehendeth it not.
And so, there are many different ways to break a sealing or seal of any type, but with some there is only a right way to break the seal and a right person to do it. Other sealings aren't to be broken at all without serious, and in many cases, eternal consequences.
And so it is with the world of the Lord, whose powers and workings are small to the understanding of man. And yet, it is by his word - whether it be by my voice or the voice of my servants, it is the same - that great things are brought up, bad things put down, and all time put in order - or sealed - in their place for however long the Lord decrees.