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.
The higher a warrior's points value, the better that warrior will be – it might be stronger, tougher, faster, have higher leadership, wear better armour, and so on.
A humble Empire Spearman is a fairly average soldier, costing around 5 points, whilst a mighty Dragon costs in the realm of 300. Most warriors 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. Knowing the points value of your models is important, as it gives you a handy way of reckoning your army's effectiveness. A horde of two hundred Goblins can be just as mighty a force as a score of heavily armoured Knights – something not necessarily apparent from looking at the models themselves. Most games of Warhammer take place between armies of an equal points value.