It is the highest number specified for any ordinary advance you have discovered, increased by the amount specified for any future technology advance you have discovered. It is multiplied by the number of modules in the proposed unit, to give the total construction cost of the unit (excluding other enhancements that are not modules but have their own cost requirements).
For example, near the start of the game you may design ground units after researching Warrior Code, which carries a cost factor of 3. A unit that has one weapons module and one armor module will cost 6 material resources to build. The discovery of Horseback Riding allows more modules but has no cost-per-module factor. You can add a weapons or mobility module or two (or three or four), or a second armor module, all at a cost of 3 each.
If you then discover Chivalry, which carries a cost factor of 5, your next unit design will cost 5 per module. If you then discover Bronze Working, which carries a cost factor of only 4, there is no increase or decrease. For that reason, you should usually research Bronze Working before researching an advance that has a higher cost.
Later you may discover advances that affect aspects of a unit other than modules. An example is Ballistics, which gives you the option of including the unit special feature Artillery in any ground or naval unit. That adds another +1 to the total construction cost.