If you have a Force Touch trackpad, you can access additional functions in GarageBand by changing how hard you press. To force click, you press firmly on the trackpad until you feel a deeper click.
In the Tracks area, force click an empty area in a software instrument track.
In the Tracks area, force click an empty area in an audio track, then choose an audio file from the menu that appears.
In the Tracks area, force click the upper part of a region, then type a new name in the name field that appears.
In the Piano Roll Editor or Score Editor, force click an empty part of the editing area.
In the Piano Roll Editor or Score Editor, force click a note.
In the Tracks area, force click the empty area below the last (lowest) track header.
In the Tracks area, force click the automation area of a region.
The trackpad also gives you feedback (you feel a subtle vibration) when you reorder tracks, move a region to a different track, adjust track volume and pan, and perform other actions.