Myopia occurs where the image is focused at a point in front of the retina - the eyeball is too long. Vision is better for near objects than for far objects.
This condition can be corrected by the use of a concave spectacle lens. Also surgical treatment may be possible with laser (photoreactive keratectomy).
Myopia, like hypermetropia, is simply a physiological variant of length (the eyeball is longer than normal), and usually ceases to progress after adolescence. There may even be a slight reduction of myopia in late middle-age.
Progressive myopia, in which the eyeball continues to lengthen throughout life, may lead to retinal degeneration or even retinal detachment.