Metronome symbol

The metronome button (symbol/icon) turns the metronome on or off (makes it produce sound or not produce sound). The number setting is how many beats per minute, how many times per minute the metronome will produce a sound.

A left-click on the metronome button turns it on or off
A click-and-drag on the number just to the right of the metronome button increases or decreases the beats-per-minute
A click (left-click) on the triangle to the right produces a drop-down menu in which you can set the beats-per-minute, the time signature (beats per measure of music), and the volume of the sound the metronome produces