When you record you’ll usually want a reference for tempo. The metronome (the triangular icon next to the volume slider in the top bar) provides an audible click for you to play to. If the icon is blue, it’s audible, if it’s white it’s not - clicking it switches it off or on.
The metronome has settings you can adjust for your needs. Open the Settings menu in GarageBand - aka “click the wrench icon” and select "Metronome and Count-In"
The Count-In switch enables the metronome to play one bar of click before the recording starts. If you turn the Count-In off recording starts as soon as you click the record button.
Visual Count-in makes the count-in visible on the screen.
There are four sounds you can select for the metronome - Click, Woodblock, Hi-Hat, and Rimshot. You can also select “no sound” which might seem strange, but with Count-In and Visual Count-In turned on you’d get a visual count off and then you’re just into your track.
The Metronome Level adjusts the volume of the metronome.
Watch the video walkthrough below: