"lavarsi" is a reflexive verb, it means someone is washing her/himself, while "lavare" means you're washing someone or something else. That "si" at the end you can see as a "-self", basically. Ciao, Massimo.

Dove è Mario? È andato a lavarsi l'auto...where is Mario? He went to wash his car by himself...forgive my english . I don t know how one says that one in english