Myopia consists of a defect in the lens, the shape of the eyeball, or in the density of the optical components of the eye. In any case, a myopic eye can focus near objects successfully onto the retina but can't focus distant objects clearly at all. The end result is a difficult time accomodating to the various demands placed on a person with myopia or near sightedness.
Objects that are distant present light rays that have become more parallel - the angle between them has decreased to a great extent. In fact, if the distance is great compared to the size of our eyeball, the light rays for all intents are parallel to one another.
Myopic eye focuses near object clearly, but "over-focuses" the parallel rays from a distant object. The retinal image is thus very blurry. So the problem becomes how to make the parallel rays look more like the diverging rays the lens can handle well
Hence, the correction for myopia has beendone using a concave lens. The parallel rays are diverged and the lens can focus them nicely