Sunlight is the light emitted by the sun, which is electromagnetic radiation consisting of visible, infrared and ultraviolet light. While the light and warmth provided by the sun is essential to life on earth, it has special meaning to the supernatural.
Vampires are vulnerable to sunlight, specifically to ultraviolet light, to the point where direct exposure will cause their skin to burn and burst out in flames. A high amount of exposure will kill a Vampire almost instantly and reduce them to ashes. Vampires may survive partial or brief exposure, though it will leave them badly burned and they will require blood to heal themselves.