A very common trope in vampire fiction is that vampires can't walk in the sun, and will die if they try to. See, for example, Suicide by Sunlight on TVTropes.

Why is that? What is the origin of the myth?

Wikipedia's Vampire folklore by region article doesn't offer an answer, and neither does the source of much vampiric lore, Bram Stocker's Dracula. Sunlight limits the Count's powers, but it doesn't kill the creature.