If you are monolingual, you speak or use only one language. A child who has only ever lived in Mandarin at home is monolingual until another language becomes part of real life, not just a school subject. The word also describes things, not only people: a monolingual dictionary is English-to-English, with no Chinese on the page; a monolingual classroom is one that runs in a single language. You can use it as a noun too: the monolinguals in the room. It is a calm, factual word — not praise, not an insult. Though actually, almost nobody is perfectly monolingual if you count a few borrowed words they have picked up; we still use it for someone whose real life happens in one language.