Someone who plays the flute. The thing worth knowing is the spelling: American English writes 'flutist', while British English writes 'flautist', with an 'au'. They are the same job and the same person. In Taiwan, school English mostly follows American books, so you will probably meet 'flutist' first, but concert programmes and classical music writing often use 'flautist' even in places that otherwise write American English, because it sounds more formal to some ears. Either spelling is correct; just be consistent within one piece of writing. The word can mean a professional in an orchestra or simply a person who plays.

長笛手、吹長笛的人（美式拼 flutist，英式拼 flautist，同一個意思）