Howdy! Welcome to the Alpha Centauri forum.
"Reverse engineering" means something like "using the unit workshop to create a new unit that you couldn't otherwise create." For example, suppose that your Probe Team subverts another faction's Impact Rover (4-1-2) and you haven't yet discovered Nonlinear Mathematics. You can load the Impact Rover in the unit workshop and change it to an Infantry chassis, thus creating an Impact Infantry (4-1-1). Another example: Suppose that you've discovered Planetary Networks (which allows you to build Probe Teams), but not Doctrine: Mobility (so you don't have the Speeder chassis yet). You can load the Probe Team in the unit workshop and change its equipment to Hand Weapons, thus creating a Scout Rover. In each case you've created a unit for which you don't have the required tech. That's what's usually meant by "reverse engineering."
Hope that helps. Please post again if you have any other questions.
Petek