The metronome is your guide to play in a steady tempo, or when you need a count-in to the part you're playing next.
When you enter the studio, the metronome is enabled by default (unless you add a loop - then it's disabled). It can be enabled/disabled by clicking the On/Off button (1) at the bottom of the studio. You can also change the volume and sound for the metronome by clicking the cog wheel next to the On/Off button!
To the left of the metronome, you'll find the Tempo. This is where you can set the tempo of your project in BPM (beats per minute). The default tempo when you start a new project is 120 BPM, which means that for every minute, you will hear 120 beats. Entering a higher value here results in a faster tempo, and vice versa.
(1) Metronome and tempo