What does the capacity mean?

It indicates how much energy can be stored in the battery. If the capacity is halved, the possible working time of the fin is also halved. More capacity often also means a larger and heavier battery.

This is stated in ampere hours (Ah), i.e. how much charge can be drawn from the battery per hour. With the same voltage, the capacity can be increased by connecting several cells in parallel. Both the voltage (V) and the current (A) are relevant for the resulting runtime of the device.