Combat trim is an automated system that helps keep you plane trimmed during flight. (if it is on). It only works within certain speed limitations (too slow or too fast and it won't help you). It assists with keeping the plane flying at the correct pitch/yaw/angle of attack.
Many players use it, many don't. I personally use it, but I've been told that learning to manually trim the plane results in better performance.
By default Ctrl X turns it on/off. If it is off and you use auto climb / speed / level, then it will turn back on if the option is checked in your flight preferences.
