An item is Bind on Equip (BoE) if it becomes soulbound when equipped by a player character. A warning appears before a player equips a Bind on Equip item for the first time, asking if the player is sure they want to do so. In other words, it may not be traded or placed on the Auction House after being equipped. Nearly all worthwhile loot in WoW is either BoE, Bind on Pickup (BoP), or Bind on Use (BoU). Note that the in-game tooltip actually displays the words "Binds when equipped", that's why some conservative players use "BwE", but "BoE" appears to be more popular.