The monument for the Mother (Azerbaijani) language in Nakhchivan, Azerbaijan

A first language (also mother language, mother tongue, native language, arterial language, or L1) often means the language that a person learns first. It helps one understand words and concepts in the style of that language.

Sometimes, but not often, first language means the language that a person speaks best (the second language is then spoken less well than the first language, etc.). In that sense, a person could have more than one first or second language.