In informal British English, brill means very good — great, excellent, just what you wanted. People say it about a day, a film, an idea, a plan: “That’s brill!” It is a short, spoken form of brilliant, and it sounds friendly and light, not careful or formal. You will hear it in the UK far more than in the US, where it may simply puzzle people. It had a strong youthful feel in the late twentieth century; some British speakers still use it, and some younger ones find it a bit dated, though it has not disappeared. Keep it for chat and messages. It is not the flatfish also called brill, and it is not a surname.