What exactly is trim?
If the aircraft is trimmed, the flight controls are in a state where no force needs to be exerted in order to continue straight and level flight. Aerodynamic and gravitational moments about all three axes are nulled out.

Changing the trim of the aircraft in pitch could happen in the following ways:


a shift in centre of gravity, for instance by shifting fuel from centre tanks to tail tanks, or payload shifting position;
a change in thrust of an engine that is located above or below the centre of gravity;
a change in horizontal tail angle or trim tab;
a deflection of the elevator.


If the aircraft was in straight and level flight and the trim changes because of a change in any of the above, it can be compensated for by the other factors. Elevator deflection is fastest but requires a pilot force which is uncomfortable for longer periods, that is why the forces can be trimmed out to make the new stick position the neutral force one.