Military points are the limit of good units you can build. The better the unit, the more military points are needed. The military points of all your units are added together. In the game bar to the left of the minimap you can see the number of military points you still have available. If the points are marked with a plus (e.g. "+1,500"), this means that you still have 1,500 free military points available.
If a number is shown with a minus (e.g. -1,500), it means that you can only build units again when the military points change to positive. To do this, you must upgrade the military headquarters in your base (or bases). For each additional expansion level of a military headquarters, your military points increase by +400.
The military centre has a total of 10 expansion stages. A fully developed military centre increases your military points by a total of 4,000 points.