In nearsightedness (myopia), the eyeball is too long - images would form in front of the retina (if they could), but there is nothing there onto which the image can be projected. So, the 'out of focus' rays land on the actual retina. You can use a concave (diverging) lens to "pre" spread out the rays, so that they hit the retina in focus.
Similarly for farsightedness (hyperopia), the eyeball is too short. Images would form "behind" the retina if they could, but the actual retina gets in the way. So, you can use a convex (converging) lens to "pre" converge the rays so that they actually hit the retina in focus.
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