The Metronome in GarageBand offers a steady beat that helps you keep time when you’re recording or just listening to your song. The feature also uses a one-measure Count-in so that you can get ready when you’re recording a song, just like you hear live bands do when performing.
You can turn the Metronome and Count-in on and off as well as adjust the Count-in bars on both Mac and iOS, and this tutorial shows you how.