Magnification is the most important specification of any eyepiece, and will tell you how zoomed in the view that your eyepiece & telescope has. Magnification varies based on both the telescope and the eyepiece you use, so it's not a constant number. We can use the focal length of both the eyepiece and telescope to determine the magnification with the following formula:
Magnification = Telescope Focal Length (mm) / Eyepiece Focal Length (mm)
A 20mm eyepiece on a 2000mm telescope (2000 / 20) gives you 100 power (100x), this makes objects appear 100 times closer to you through the telescope than they appear to your unaided eye.
Note: When using your telescope at different powers, you generally have a choice of a small, sharp, and bright image at lower magnification; or a larger, yet blurred and dim image at higher magnification. The reason being is that the telescope gathers a fixed amount of light, and at higher magnifications, the same amount of light is being spread over a larger area, resulting in a dimmer image.