Flat is a super common word in English with multiple meanings, depending on the context.
1. As an adjective: having a level surface; not curved or bumpy
2. As a noun: a set of rooms for living in, usually on one floor of a building (British English)
3. In music: a note lowered a semitone below natural pitch
4. In slang: completely or absolutely (e.g. "flat broke")
5. In automotive: a flat tire (American English)