What is a battery capacity?

A battery's capacity (Watt * hour) is the amount of energy it can store.

Knowing the power (in Watts) consumption of the equipment allows you to roughly estimate how long the battery will last: Time = Energy / Power.

As an example, if you are using a 160Wh battery to power a 60W RED camera, the time the battery is expected to last is 2.67 hours or 2 hours and 40 minutes (160Wh / 60W = 2.67h).