This turns the metronome on or off. When painted the metronome is "on;" if it's just an outline it's "off." The metronome doesn't actually play, though, until you use the "Hear" command (or until the program plays something for you). What this really means when it's "on" is "add the sound of a metronome when playing music."
The metronome will play at the currently chosen tempo, and will play in time with the music (that is, the metronome will click on each downbeat).