An item is Bind on Use (BoU) if it becomes Soulbound when first used. This label is for items that are used from the player's bags without being equipped. A warning appears before a player binds a Bind on Use item by using it for the first time, asking if the player is sure they want to do so.
Early in the game's history, almost all mounts and companions were bind-on-use. Such items have now been converted into consumables that grant abilities in the Collections window. They, as well as other consumables that grant permanent benefits such as recipes, are effectively bind-on-use since abilities can't be traded; they just don't remain as soulbound items in the inventory anymore.