The front clear window is called the cornea. The cornea and lens of the eye normally work together to focus a clear image onto the retina (the sensitive, seeing layer at the back of the eye). If there is an imbalance between the length of the eye, the curvature of the cornea and the lens, the image seen by the retina is not in focus.
A lens works by bending light rays to a point of focus, a process called refraction. The focus power of the glasses or contact lenses required to change the focus of the eye in order to see clearly is called the “refractive error” or spectacle error. It is measured in units called diopters and given as a number.