What is trimming the fuel tank? In a recent news article on an Airbus A400M crash, it has the following line:


  Sources have told Aviation Week that aircraft MSN23, destined for Turkey, featured new software that would trim the fuel tanks, allowing the aircraft to fly certain military maneuvers.


I understand what trimming is, but what does it mean to trim the fuel tanks?
Just like normal trimming. However trim the fuel tank use aft fuel tank to achieve the trimming effect. Fuel is pumped between main tank and aft tank to shift CG around to a desire position during flight. 

Fuel tank trimming has additional benefit of keeping the airplane in "clean" configuration, creating less drag compare to normal trimming.