Diffraction Limited Optics
Many telescope makers boast their products have Diffraction Limited Optics.  What does that mean? To understand the meaning requires only a little Physics. When a beam of light passes through a circular opening (aperture), it spreads out a little.  This phenomenon is called diffraction.  As the aperture or circular opening gets wider the spreading diminishes,- thus wider openings have less diffraction.
The image of a point source of light (a star) after diffraction appears to be a central circular spot surrounded by a series of dark and bright rings. This spot is called an Airy disk.
Where d = diameter in millimeters, l = the wavelength of light used in meters, and N = the telescope's Focal Ratio:
d = 2440 l N