A telescope collects light and forms a small fixed-size image at a point
(called the prime focus) that's determined by the focal length of the
optical system. You can see this image by aiming your telescope at
something bright, such as the Moon, taking out the eyepiece and star
diagonal, and holding a piece of paper behind the focuser. Move the
paper back and forth. At some point, you will find a small, but sharp,
image of the Moon projected onto the paper. This is the prime focus
image formed by the telescope. Unfortunately, human eyes typically
cannot focus sharply on an image unless it's more than eight inches from
the eye. This makes it difficult to see detail in the small prime focus
image formed by the telescope if it's examined solely with the unaided
eye. An eyepiece is a small microscope that allows you to get closer
than eight inches from that small fixed-focus image -- and the closer
you can get to an object, the bigger it appears. A 25mm eyepiece, for
example, lets you focus on the scope's prime focus image from an
effective distance of only 25mm (one inch away from your eye); a 12mm
eyepiece puts you half an inch away; etc. The magnification of an
eyepiece is found by dividing the telescope focal length by the eyepiece
focal length. A 25mm eyepiece used with a 2000mm focal length scope
therefore provides 80 power (2000 / 25 = 80x), making objects appear 80
times larger than they do to the bare eye (or 80 times closer, to put it
another way).