The meaning of Ah in battery terms is Amp Hours.
The A stands for Amps and the h stands for hours.
So for example if a battery says it has 100 Ah it means the battery has 100 Amp Hours.
An ampere hour or amp hour is a unit of electric charge, having dimensions of electric current multiplied by time, equal to the charge transferred by a steady current of one ampere flowing for one hour, or 3,600 coulombs.
The commonly seen milliampere hour is one-thousandth of an ampere hour.
As the name suggests this means how many amps the battery can deliver in an hour.
For example, a 12V lithium battery with a capacity of 100Ah can deliver 100A to a 12-volt device for one hour.
The same 100Ah battery could supply power for 4 hours (100/25=4) to a 25 ampere device.
One ampere-hour (or amp-hour or Ah) is a current of one ampere flowing for one hour.
The amount of charge transferred in that hour is 3,600 coulombs (ampere-seconds).
A milliampere-hour (mAh or milliamp-hour) is a thousandth of an amp-hour.
An amp-hour is one amp for one hour, or 10 amps for 1/10 of an hour and so forth. It is amps X hours.
If you have something that pulls 20 amps, and you use it for 20 minutes, then the amp-hours used would be 20 (amps) X . 333 (hours), or 6.67 AH.
The key is to use the watts you know to calculate the amps at the battery voltage .
For example, say you want to run a 250 watt 110VAC light bulb from an inverter for 5 hours.
Amp-hours (at 12 volts) = watt-hours / 12 volts = 1470 / 12 = 122.5 amp-hours.
What is the difference between amp-hours and watt hours?
Put simply, the power of an amp-hour (or, thus, mAh) depends on the voltage, whereas a watt-hour is always a watt-hour.
Other variables and terms, such as cycle life and C-rate, also affect the real-world available energy in a battery, but using watt-hours at least eliminates the ambiguity of the amp-hour.