A laptop's battery life is the total number of hours that a laptop can work on battery power without needing to be plugged in. The battery life is usually quoted as an approximation rather than an exact time period, as computing power and efficiency changes from one laptop to another.
To calculate the approximate length of time that a laptop will run on battery power before it needs to be plugged in, you can use this handy equation:
(battery capacity) x (percentage charge) / (average energy usage per hour) = time until depletion
So for example, if your laptop has a 100% charge and you only use it on average 3 hours per day then you would have some 16-17 hours of usage before it would need to be plugged in.