The mm in a telescope is the same thing as the mm in your eyeglasses.
The lens in a telescope magnifies the image of the object being viewed. The magnification can be set by multiplying the focal length of the objective lens by the focal length of the eyepiece lens. For example, if the objective lens of a telescope has a focal length of 400mm and the eyepiece lens has a focal length of 25mm then the magnification is 400X25=10,000mm.