Myopia
Myopia or short sightedness is caused by the eye being too long or too powerful. This leads to the light being focussed in front of the retina. Giving a blurred image especially of things in the distance (whilst things close may be very clear).
In order to give a clear image a lens that reduces the power of the eye is used to bring the image onto the retina. As the lens reduces the power if the eye it is a negative lens hence the minus sign in front of the number.
Myopia is caused by light be focussed in front of the retina, negative lenses are used to correct it
Short sightedness usually begins in childhood and as the eye grows may get worse. After the age of about 20 the eye does not grow anymore and the prescription may level off.
One advantage of short sightedness is that often in middle age short sighted people can continue to read without glasses.