It is the number reached by adding to the initial basic strength all the numbers specified for the advances the player has discovered. It is then multiplied by the number of weapons or armor modules in the proposed unit, to give the total attacking or defensive (respectively) strength of the unit.
For example, near the start of the game you may start designing ground units right after researching Warrior Code, which carries a strength factor of 4. A unit that has one weapons module and one armor module will have basic attack strength of 4 and defensive strength of 4. Incorporating in the design a second weapons or armor module gives you a strength of 8 for that feature. If you then discover Bronze Working, which adds 2 to ground unit strength, each module in a future design (before you discover any other ground-strength-enhancing advance) will have a strength of 6. If the only other relevant advance you then discover is Monotheism, which adds 3, your next design will have strengths of 9 per module.