Ever notice sometimes when you take your hand of the stick, your airplane wants to climb, if you've got full power, or dive if you're throttled down?
Trim gives the pilot a way of putting a little control pressure on the airplane to "trim out" those tendencies.
For example, on a little Cessna trainer, you don't have 2,000 horsepower like you do in a Mustang. To climb, one often "trims" the elevator up a few notches to produce a slight, gradual nose-up attitude.
Planes either have "trim tabs," little sections of each control surface (rudder, elevator, etc) that move when he moves the trim wheel or the trim wheel actually moves these surfaces slightly when it's turned.
In WWII Fighters, you have a "auto trim" option (S, I think). If you hit this when cruising, your plane should track nice and steady. But in combat, you want to turn this off.
Does that help?