The objective lens is the one on the opposite end of the scope from the one you look into. That is, it's the one closest to the "object" you're looking at. The one you look into is called the ocular lens.

An adjustable objective lets you adjust out parallax at the distance you select. Parallax is the apparent movement of the reticle in relation to the target as your head moves around behind the scope. This is because the image of the reticle and the image of the object you're looking at are in two different opticalplanes.For example, if you set your Adj. Obj. for a distance of 150 yards, then look at an object at that same distance, then the reticle (crosshairs) will stay put on the same spot even if your eye doesn't stay exactly in the same place behind the scope. If you look at an object at a different distance, say 300 yards, andwhile doing somove your head from side to side, the reticle will move slightly in relation to the target and not stay on the exact same spot.

Hope that helps explain it.