think of trimming your flight controls like having a front end alignment on your car. If your front wheels are out of alignment, you are continuesly have to make corrections. For example if your car is always pulling to the right you have to hold left turning pressure on the wheel to keep it going straight.
Same thing with an airplane. It an airplane is out of trim, it will want to turn or climb or dive and you will be fighting that with your joystick all the time. By trimming, you are able to fly 'hands off' and the airplane is less inclined to wander about the sky.
Hope that helps.
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