Bind on Pickup (BoP), also known as Bind on Acquire (BoA), refers to an item which becomes soulbound when picked up (looted from a corpse or container, received from an NPC as a vendor purchase or quest reward, or crafted). A warning appears before a player loots a Bind on Pickup item, asking if the player is sure they want to do so. This also applies for rolling on a BoP item (either Need or Greed) while using Group Loot.
Crafted items that are Bind on Pickup can only be acquired by making them yourself, although it is sometimes possible to drop the crafting profession after having made any BoP items you want from it. However, some of these items require a certain skill level or specialization in order for you to equip them.
Note that the in-game tooltip actually displays the words "Binds when picked up", that's why some conservative players use "BwP", but "BoP" appears to be more popular.