The Coffin is a block added by the Witchery mod. It is used by Vampires to sleep through the day, as a result of their weakness to sunlight. They operate very similarly to Vanilla beds, in that the player's spawn point will be set on sleep, and for the player to sleep on a multiplayer server all players must be sleeping. Taglock Kits operate exactly the same on coffins as they do Vanilla beds.
Coffins can be used to sleep through the day, as well as the night.
In order for the player to use it, they must first open it with the sneak-use key. Then, they can sleep by using their use key on the block.