What does a 12V 7Ah C20 rating mean?

12 V is the nominal voltage; 7 Ah is the capacity measured over a 20-hour discharge. Discharged at 350 mA for 20 hours the battery delivers its rated capacity.It does not mean you can draw 7 A for one hour, the one-hour rate for that battery is closer to 3.6 A, because faster discharge extracts less total energy.