An important part of basic trimming of your airplane includes establishing a nice glide slope that is primarily used during landing. Let's say that you are on your downwind leg, straight and level at your cruise speed, you then drop the throttle to a couple of clicks above idle - the resulting descent angle is your glide slope. If it's too steep and fast you'll need to adjust it using up elevator to hold a nice slope and reduce speed. If it's too shallow and your controls are "wallowy" you'll need to adjust it using down elevator to increase the angle of descent to regain speed and control.