Myopia simply means short sight. This Myopia happens when light from an object forms an image before it reaches your retina. This could happen because your eye is too long, or your cornea or crystalline lens bends the light too steeply. A person with myopic vision can see clearly when looking at objects that are close to them, but has blurred vision when looking at distant objects.
If you place a concave lens in front of a myopic eye, the image moves back to the retina so it is clear. The result is that a myopic person can then see objects that are both close to them and in the distance clearly.