The Lockpick is an item that appears in certain games in the Fire Emblem series. The Lockpick item allows certain characters throughout the game to open both Doors and Treasure Chests without the character requiring a Door Key or Chest Key.
The Lockpick item was used in many of the original Fire Emblem games. Seeing as it allowed a Thief, Assassin, or Rogue to open Doors and Treasure Chests without using a Door Key or a Chest Key, both of which can only be used twice before they disappear from a character's inventory. This made it a very valuable item to have.
Lockpicks are very rare to get in the Fire Emblem games they exist in. One always comes with the first Thief unit a player receives in the game so that the Thief can pick locks right from the get go. Other than that Lockpicks can only be purchased in Secret Shops in the games and the only other way to obtain them is to use your Thief, Assassin or Rogue to steal a Lockpick from an enemy Thief. This is usually not that difficult since non-player thieves are often quite weak and don't pose a threat to any player controled units.
The actual Lockpick item has 15 uses/durability in the majority of the games. So this allows a player to open a total combination of Doors and Chests equaling 15 before the Lockpick disappears from a character's inventory and becomes useless.
In Fire Emblem: The Sacred Stones the Lockpick was only useable by the Thief and Assassin classes. The Rouge class had the ability to open Doors and Chests without a Lockpick. In that game there are two possible Rogues, Rennac (a pre-promoted Rogue), and Colm (a thief who could be promoted into a Rogue or Assassin).
In the game that came after Fire Emblem: The Sacred Stones, Fire Emblem: Path of Radiance, the Lockpick was removed from gameplay. In Path Of Radiance there was two Thieves, Volke and Sothe, (Volke promoted into an Assassin and Sothe didn't promote), both did not need the Lockpick item to open Chests and Doors they could without them. This carried on into Fire Emblem: Radiant Dawn, the sequel. It is unlikely that the Lockpick item will be re-introduced in future Fire Emblem games (except possibly re-makes).