A whistle is a small wind instrument, and you probably know people who are good at whistling without an instrument. Trains have whistles, and the wind whistles too. The world is full of whistles.
Whistle is a word with several meanings, but they have in common a similar high-pitched, airy sound. This is different from most instruments. You can't tuba without a tuba, but you can whistle without a whistle. Sometimes people's teeth make a whistling sound by accident. Referees and traffic cops carry whistles to get people's attention. Other people whistle for many reasons: because they're impressed, see something they like, or just want to say, "Hey you!"