What is meant by short-sightedness?

Hint
As the name implies, short sightedness is a condition in which the object in long distance appears blurred while the objects in short distance look clear. This is a very general issue with human eyes as the lens of the eye loses its flexibility to adopt different focal lengths.

Complete step by step answer
We will see about short-sightedness and how it is caused.
Short-sightedness technically known as myopia is an error in vision in which one can focus the nearby objects clearly but the distant objects appear blurred. It is due to the refractive error in the eye which is clearly explained below.
Human eye consist of two parts to focus objects,
The cornea: It is dome shaped. It is the outermost layer which controls and focuses the light into the eye. When a light enters the eye, first it strikes the cornea, the cornea bends or refracts the light .
The lens: It is the inner layer of the eye which focuses the light that passes through it to the retina which converts light into neural signals and sends it to the brain for visual recognition.
Normally these cornea and lens have a smooth curvature like a marble that bends all the incoming light to make a clearly focused image on the retina. If either cornea or lens is not curved properly then a refractive error will occur in the eye resulting in blurry appearance.
Myopia is caused mainly when eyeball is longer than normal or when the cornea is curved too steeply this results in focussing of light in front of retina instead of being focused precisely on retina which leads to blurry appearance of distant objects.

Note
Short-sightedness or myopia is not a disease but a refractive error. It is inheritable. It has a high risk of heritability in first degree relatives i.e. children or siblings. If both father and mother have myopia then there is a high chance for their children to get myopia.