Almost every model will have a points value given within its profile. Points values reflect a model's worth within its army.
Most models have a basic points value that increases as various optional items of equipment (weapons, armour, magic items, for example) are added. By adding together the points values of all of the models you have selected, you find the total points value of your army. Knowing this enables players to play evenly matched battles.
Points values and the rules for building an army are explained in greater detail in the Warhammer Armies section.