Just like everyone said before me, they are heating pads that are under all the tanks and around the grey/black gate valves. They run on electric and wouldn't depend on them entirely to keep your system freeze-proof. You need to make sure you run your furnace, which the bathroom duct runs through this compartment and radiates some heat within. When it gets below 20F, I typically also keep the water heater on all the time. It'll also add some heat down below. Last winter, it got down to single digits over night and we did fine.
2015 FR3 30DS