Yes it is milliamps per hour...sometimes referred to as milliamp hours...etc.

Basically it means that it can produce that amperage for a period of 1 hour.
Example. A 1000mah battery can produce a constant 1A(1000mA) current for 1 hour.
You can split amps up and such......

Example. You can take a 1000mah battery and make it produce a constant 2A current for 30 min.
or 4A for 15min or 8A for 7.5min....did you catch that?

So why cant you pull 50,000A really fast....well because the chemical properties of the battery wont allow that fast a transfer of current without generating heat.....which is why our lipos get warm when we suck lots of juice from them. This is also where "discharge" rate comes into play. This tells us how much current we can pull from the battery before it starts losing current as heat.

So if you know what the current draw is on your system. You can figure out what mAh battery you need to attain the flight time you want.

So I know what your thinking. Most brushless motors draw 8-10A, how are people getting 25min out of a 730mah battery?....well simple......we dont fly around at full throttle all the time. This is where piloting comes into play....Learn how to manage your available power, and you can increase your flight times simply by conservation.