Myopia (also known as nearsightedness) is a particularly common refractive error which causes patients to have difficulty clearly seeing distant objects. Like most refractive errors, it’s caused by the shape of the eyeball.
In a correctly functioning eye, light travels through the cornea, which bends it at the appropriate angle to come to a focal point on the retina. In a myopic eye, the eyeball is too long in proportion to the curvature of the cornea. This results in the light coming to a focal point just in front of the retina, rather than directly on it.
Some cases of myopia progress, getting worse and worse over time. This is called malignant myopia.