The elevator (smaller wing on the back) points up or down, pushing the air up or down, which in turn raises or lowers the tail/nose of the plane (it's pitch).

The elevator (smaller wing on the back) points up or down, pushing the air up or down, which in turn raises or lowers the tail/nose of the plane (it's pitch). Some planes have little cutouts on the back of the elevator that move, thus deflecting the wind. Other planes, the whole back wing moves (stabilator). To keep the nose pointed up or down, it can get tiring to constantly apply force on the pitch control. So now we can "trim", which changes the neutral position of the elevator to be higher/lower. Some planes have little cutouts on the elevator or stabilator for trim. Other planes, the whole elevator / stabilator moves for trim. Some times, when the whole elevator / stabilator moves up/down for trim, you need a lot of force to move it back down/up, especially if its neutral position has changed. Occasionally, under high loads due to wind/pressure, you just can't. So you have to go to it's current neutral position (no matter how high or low) so you're not fighting a loosing battle. Then you have to reverse the trim. Then you can apply the opposite force.