It’s the current capacity of the battery. The maximum current the battery can be discharged at continuously for 1 hour. For example; you can draw 10A continuously from a 10Ah battery for 1 hour or 5A for 2 hours.
This is a parameter that gives you a range. However, the energy density of a battery should be measured by Wh(watt-hour) if you are comparing battery systems of different voltage. Ah increase when using higher capacity Li-Ion cells or using more Li-Ion cells in parallel in a battery pack.