A Guitar Loop is simply a short recording captured by your looper pedal which is then played/looped back through your amp, over and over.
A Guitar Loop consists of a Primary Loop and Layers:
The Primary Loop
The primary loop is the very first recording your looper pedal captures in a performance. It’s the most important because it’s the recording that determines the length and time of your loop.
Layers
Layers are simply additional guitar parts recorded over the top of your primary loop. You can create as many layers as you want providing you don’t run out of loop recording time dictated by whichever looper pedal you’re using.
Your first recorded layer is called Layer 1, your second Layer 2, your third Layer 3 and so on.