Sunlight is ultraviolet light from earth's sun that serves one of the main weaknesses of a vampire.
Vampires: After Count Dracula was transformed into a vampire, the curse was powered from the earth's darkness at night, making the sun a weakness. The reason this is because the sun is a symbol of life, and a vampire is undead. Although the sun didn't effect Count due to the fact that he didn't die, it however effected whoever he turned, including his brother Vladimir, and therefore all vampires in existence.
Theo Quinn: Theo is immune to the effects of the sun, being Count Dracula and Louise-Anne Dracula's biological son. Because Count Dracula was immune to sunlight, this means Theo is too.
Dhampir: Being half-human/half-vampire, Dhampir's human side allows them to be impervious to the power of the sun.