Angel: Vampires don't sleep in coffins. It's a misconception made popular by hack writers and ignorant media. In fact, you know, we can and do move around during the day. As long as we avoid direct sunlight. Got it?
Angel is refering to the many horror authors who portray vampires as sleeping in coffins during the day. Perhaps the most notable is Bram Stoker's novel Dracula in which the vampire sleeps in a coffin filled with his native soil.