That means when you enlarge an image in a telescope, at some point you will make the full image resolution visible and further magnification only makes the larger image dimmer and blurrier. Or, to put it another way, when the telescope's image is magnified to the point where it has less resolution than your eye, it will always look fuzzy and blurry. The larger the telescope, the more you can magnify the image before this happens. A similar effect can be seen when enlarging photographs. What you will see in the telescope-, the field of view, is a function of its diameter and magnification.
Telescopes have focal ratios based on diameter.
The magnification a telescope produces is its focal length divided by the focal length for the eyepiece being used. For example, a 20mm focal length eyepiece is referred to as a 20mm eyepiece. The lower the magnification a telescope operates at, the wider the field of view. Telescopes with very short focal ratios also tend to have fish-eye type curvature in their images. Thus, low focal ratio telescopes are typically seen as only fit for low power use for nebulae and galaxies.