mAh's are milliamp hours, or basically the amount of potential energy stored in a battery. For example, using the same coil and the same
vaping, a 2500mA battery will last longer than a 2000mA.
There are also two types of passthroughs, one which actually plugs you into the current and one which allows "charge while using." e.g., the itaste VV and MVP allow passthrough, where you can use it while it's charging -- but if the battery is too low, you have to wait till it charges up a little.