In a loudspeaker system it fills the highest frequencies, on average a tweeter operates in a frequency between 3000 and 20,000 Hz, but you can push much higher in the high-end models.
It is normally built with light materials that allow a quick response. Moreover, the type of frequency reproduced makes it a very directional sound, so it is necessary to orient it towards the listener.
This category also includes super-tweeters that go even higher in frequency.