The metronome helps to keep time and will play a sound on each measure and beat.  To turn the metronome on or off, go to the Project Details Tab and toggle the Metronome button.  Alternatively, you can click the Mix menu followed by clicking Metronome (a checked state indicates that it is on).  The easiest way to toggle the state of the metronome is to click the metronome icon on the Master Bar.

Customizing The Metronome
You can customize the metronome's sounds and volume level by going into the Preferences and choosing the Metronome page. 

Play Every X Beats
By editing a marker, you can also instruct the metronome to play every Nth beat (it defaults to playing every beat).