When you are sleeping in a traditional ground tent, the ground itself acts as a bit of an insulator between your mattress or sleeping bag and the outside air. But when you are in a rooftop tent, the bottom of the tent is exposed to the air. So changes in temperature can cause moisture to build up between the bottom of the tent and the mattress. The Anti-Condensation Mat helps by adding some ventilation between the mattress and the tent floor keeping the moisture at bay.
AER (2-3 P), AER Maxx (4-5 P)