What is Farming?

And how do I do it?

farming is simply what is referred to when you attack someone to steal their kuwal.

to do that you need to build a strike, train attack units and buy attack weapons.
then you find an appropriate target in the battlefield who has less defense than your strike then you click strike mission and initiate an attack mission (you can spy on people by clicking covert mission and sending in spies for a reconaissance mission
if you can see someone's kuwal out instead of "???" it means the recon mission will be a success, if you see ???'s then either train more spies or go find yourself some other target)