It is capacity - 2300maH should be able to provide 1A for 2.3 hours, an 1100maH battery will only provide the same 1A for 1.1 hours.
These figures tend to be fairly 'generous' and are measured at the optimum loading, I would suggest connecting a torch bulb to them (of the correct voltage of course), and measuring the current taken. Then simply time how long it takes for the bulb to go dim - multiply the time (in hours) by the current (in mA) to get the maH value. But don't expect it to be the same as the rated value, for one thing the current of the lamp will change as the battery discharges and it's voltage falls.