A douche is a type of hygiene product women use to clean their vaginas. Douche is also a common slang word for "jerk": the variation douchebag is extremely popular.
There’s no clear way to define what a douche is, though anyone acting arrogant, jerky, or nasty might be described as one. Since this is a slang word that some find disgusting and particularly offensive to women, be careful when using it. The d-word isn’t as bad as the f-word, but your mother or your teacher probably doesn’t want to hear about “That douche in my math class!” So it's probably best to keep your vocabulary douche-free.