The term "daywalk" refers to a vampire's ability to somehow overcome or become immune to the effects of sunlight. The setting in which the vampires live generally dictates what steps are needed to daywalk. The process of daywalking can include: drinking fairy blood, being half-vampire and half-human, drinking a magic serum, or casting a spell. It could also be an innate ability possessed by the vampire from the start.
The "Daywalk" ability is also a reference to the Blade comic books and movies.
See also: Sun Gods Event