What is a Fireplace Surround Rebate?
If the fireplace is being fitted against a flat wall then the full depth of the fire needs to be recessed within the fire surround. However, if the fire surround is being fitted against a chimney then the depth of the fire can be recessed within the cavity of the chimney.
The rebate is the distance from the back of the wall to the front of the panel. To work out the fire recess, you subtract the rebate by 20mm to accommodate the thickness of the panel. Hence 100mm rebate gives you 80mm fire recess depth (100mm -20mm for the back panel).
The below video helps explain everything with images and certainly should you get your head around what a Fireplace Rebate Is.