mAh stands for milli Ampere hour or milli Amp hour. It is a measure of a battery's energy storage capacity. If you think of a battery as a small fuel storage tank, which in a sense it is, mAh a measure of how much "fuel" the battery holds. With a battery the higher the mAh rating the more electrical energy it can store.

Different types of batteries use different methods to measure mAh so comparing the mAh rating of one type of battery, say an alkaline battery to another say an NiMH battery, is not always meaningful.

However, in general, the mAh rating of a battery is a quick way to compare the relative energy storing capacity of one battery to another battery of the same type.