Farming is the term for attacking another player in order to take his credits. When a player does so, he uses his attack units to take the other players currency by force. If the attack is more powerful than the defense, the attacking player gets a certain amount of the defending players currency transferred onto his account where he can spend them as he likes, but if the defense is stronger than the attack the attackers force is repelled and he gets nothing. But, to attack other players you need Action Turns (a.k.a. AT). You choose how many AT you want to use to attack the other player. You can use from 1-100 AT to farm and of course the amount of currency you get is based on how many AT you spend to attack (if you use 100 AT you get 60%-90% of the other players currency). Also, when you attack, your and the defenders units lose health that can be restored in the military menu.
Here is an example of how to farm another player:
Player A has 10 quad on hand and player B sees it
Player B goes to the player A's profile and clicks the button that says 'attack'
Then player B types how many AT he wants to use and presses 'attack' again
Then player B gets a battle report on his screen that shows how many units he sent and how many units the opponent has trained in defense. Below that he sees the total power of the attack force and the defensive force. He also sees how much health your units lost during the fight. And finally at the bottom of the page he sees how much currency he stole from player A (if he managed to beat his defense that is).