With lunar photography you want to see as much detail as possible, so you have to use the maximum resolving power of your telescope. In most cases this is not possible, because the focal length is to short. A 80 mm telescope requires a few meters. It is of course possible to make such a telescope, but it is to difficult to handle. Another method to achieve this is to use oculair projection. An oculair will be placed at the focal plane. The image is now projected by the oculair, and because it enlarges the angle of the incoming light, the projected image will also be larger. So your telescope will look like a telescope with a longer focal length. The focal length of your ‘new’ telescope is called: effective focal length. This is the length you would have needed without oculair projection.
The focal plane of the objective is called: primary focus.