Farming is the process of repeatedly attacking someone 3-XXX times to earn profit,in simple terms,it's attacking again and again until you feel like stopping.
There are two kinds of farming,inactive farming,the most used method of farming,is finding a player(suitable to your kcs) that no longer plays the game,then attacking him,without returned hits. Active farming is when instead of finding inactive people,you just go to the People list from the home screen,and choose a suitable foe,this may result in returned hits which in turn results in money loss