Trimming just means you don't have to hold constant pressure on the stick(or rudder), you can fly "hands off" without holding back the stick in a climb.
Fly fast without pushing forward on the stick.
Rudder trim has a lot to do with throttle setting and speed. You can watch this with the slip/skid gauge(the curved glass tube with the ball inside)
Flying straight, the ball is in the middle. If it isn't, you're not flying straight.
Another thing about trim.
Your elevator trim works just like a cruise control and is the most important aid to precise flight control.
The guy bobbing around in formation is the guy who's always fighting the stick because he hasn't trimmed the plane.
Elevator trim sets the SPEED your aircraft will fly at. If you get the plane flying level at 350K and adjust the trim so you can let go of the stick it will always want to fly at (or close to) 350K.
Cut the throttle and it will go into a dive... and stabilize at around 350K.
Add full throttle and it will climb ..... at around 350K.
These are fighters and not designed for stable flight, they have huge engines trying to twist the plane in all kinds of directions at once so they don't react to trim as nicely as your passenger planes of today.
Very handy when landing.
Most beginners just chop the throttle, point the nose at the runway and get there at 450K. Not pretty.
Get off the end of the runway at 700m or so, cut the throttle and hold the nose up, slow the plane down to your approach speed. 50K or so above stall with gear and flaps deployed and use elevator trim for hands off flight.
Now you adjust your sink rate to the strip with throttle. If it looks like you'll be short DO NOT pull back on the stick, this will make you sink faster and fall even shorter. Just add some throttle.
When you almost touch down then you pull back the stick to hold the plane off the runway(flare) and let the speed bleed off.
When the plane is ready it will stop flying and drop the last foot onto the runway without the embarrassing crow hopping.
I find in the game it's a good idea to add some throttle just before you flare, these planes are heavy and it makes the landing smoother.
When you get some experience you'll know how far you have to be off the runway so you can glide in with your engine at idle.