This door wants to eat me! What will happen if I go in? What is this sorcery?
When you pass through that door, in either direction, you take half a heart of damage. On the other side you will usually find red chests, which usually have better stuff than normal chests (like two pills or an item), but sometimes have bad stuff like enemies or troll bombs that follow you.
See also: Curse Room (Binding of Isaac wiki)