Combat trim uses a software subroutine within Aces High to try to automate trim settings. If you leave it on, your trim controls will generally be pretty close to what they should be anytime your airplane is flying without flaps and within predictable parameters. As a beginner, you will benefit from Combat trim. Most beginners leave it on all the time.
There are some legends about Combat trim helping you to turn tighter (or preventing you from turning tighter). Ignore those stories: combat trim just helps you keep your joystick centered.
But because it is a logical, programmed tool resident in the software, it isn't very good a handling illogical, uncontrolled flight. When you are approaching a spin or a stall, flying with full flaps, or pressing your speed or climb outside of normal parameters (commonplace in combat), it will fight you, because it will be trying to return you to routine, controlled flight. Advanced players eventually learn when to turn it off. But that will come later. For now, I'd leave it on.