Blocks or monsters may sometimes drop unidentified items. These items have their data completely hidden and will require the player to identify it in order to be used. The only actual info players have on unidentified items is the item material.
Players can identify their items by drag & dropping a consumable onto their item. This consumable must have the Can Identify option toggled on. In parallel, /mi unidentify unidentifies the item they are holding. They will hear a level up sound and will receive a chat message saying their item was successfully identified.