The battery capacity is a measure of the charge stored by the battery, and is determined by the mass of active material contained in the battery. The battery capacity represents the maximum amount of energy that can be extracted from the battery. This is expressed in kilowatt-hours, or kWh.
Things are a bit complicated by the fact that some EV makers list net (or "usable") capacity, others list gross (or "total") capacity, and others still give both numbers. Gross capacity means the total installed capacity, while the net capacity is the amount that the car will actually ever get to use. The difference between net and gross capacity is there for buffer purposes. We list any and all numbers that manufacturers provide, but unfortunately we can't have all numbers for all batteries unless the companies decide to make them public.
The higher the battery capacity, the more range you can get, all other things being equal.