台子 is a colloquial noun for a raised flat surface, like a platform or a stage. It's less formal than 平台 (platform) or 舞台 (stage), so you'll hear it in everyday speech rather than in written or formal contexts. The reverse translation test can be tricky — if you see 'platform' in English, 台子 is not always the right choice; use it when the context is casual or concrete.