Explanation :
There can be many eye defects in the human eye. Some of them are Myopia, Hypermetropia, Astigmatism etc.
In Myopia the size of the eyeball is too long and the image is formed in front of the retina. A person suffering from this defect is not able to see far objects clearly but can see nearby objects normal.
Concave lenses are used to correct this defect so that the image is focused on the retina.