It's a metronome buss which is similar to a track.
If it doesn't already exist you can create one by using the "insert stereo buss" option found by right clicking in the buss pane. Shift+B shows or hides that (in the track View)
Call it metronome and make sure its output is assigned to your interface main outs.
You can then go to Preferences-->Project--->Metronome and make sure that the metronome is assigned a sound and also outputs to the metronome buss you've just created.