Optical aberrations are just an elaboration of the basic idea of defocus.
In fact, defocus is the simplest of all aberrations in which all of the rays of light come to a perfectly good focus, but that focus is not in the plane of the retina. Consequently, when the rays of light intersect the retina they are spread out evenly over an area. The reason this simple aberration is so easy to fix with spectacles is that spectacle lenses shift the focus plane forwards or backwards until the perfect image lies on the retina. By contrast, an aberrated eye will not be able to form a perfect image at any distance and therefore aberrations in general cannot be corrected by ordinary spectacle lenses or contact lenses. There are many reasons why an eye's optical system may be aberrated.