The battery run time is essentially a measure of how long your battery will last for.
Most manufacturers will list a minimum run time, which is how long the battery will last with the light on its brightest setting, and a maximum run time, which is how long it will last on its most economical setting.
These lights will also have an indicator that will tell you when the light is fully charged, and also when the battery is running low. This is often a traffic-light LED light system.
