what are these words in spanish?se me te??when do we use them?
They are pronouns. They mean different things in different contexts, but in your example se is a reflexive pronoun. This article has some very good information.
whay cant we say : La niña lava la cara?
You can, if she is washing someone else's face. The Se tells you that she is washing her own face.
"la niña lava la cara" translates as "the girl washes (the) face". So, "se" it's telling you that it's her's, she's doing it to herself.