Remember when you were a child and you played with a magnifying glass? The glass focused light to a point because its surface was smooth. Imagine the surface was instead pitted or grooved or ridged. Light would no longer focus to a point, it would be smeared or distorted in some unpredictable way, depending on the exact irregularities in the glass. These are called higher-order aberrations.
The same is true of a human cornea that has been damaged by eye injury or by refractive surgeries like Lasik or PRK. If these corneas are not regular, light no longer focuses sharply on the macula, even with glasses or contact lenses. In this case, a scleral lens can restore vision by acting as a prosthetic cornea that provides a smooth anterior surface.
The image below illustrates the reduction in higher-order aberrations before and after a scleral lens.