Every model in Warhammer has been assigned a points value, given in the relevant army list. Points values reflect a model's worth within its army.
Most models have a basic cost that increases as you upgrade their equipment and abilities, giving you a wide range of options that you can choose to spend your points on.
By adding together all the points costs of the warriors you have selected you can find out the points value of your army. Most games of Warhammer take place between armies of an equal points value.