Sealed Items are items, armor or weapons that have extra stat boost(s). It is unknown what particular stat boost(s) an item has until it has been unsealed. Sealed items can be worn while sealed, but the stat boosts wont take effect until the Item's Seals has been broken.
A sealed weapon with 1/3 seals broken. The added bonuses are "Max HP 80, Melee Max Attack 23"
Unsealing Items
To unseal an Item, a player must go to a Bartender NPC and with the necessary items.
Players can find out what items are needed to Unseal an item by speaking to a Bartender NPC and selecting to unseal, putting the item from inventory into the middle box of the window that pops up, then selecting "Unseal".
Usually the requirements for unsealing are a certain amount of Gelt and a Seal Dissolution Pearl. If the player does not have all required items, he/she can usually obtain them by either fishing or buying the item off of another player.
Having all of the requirements usually unseals all seals on the item.