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The exit pupil is width of the beam of light leaving the eyepiece. It’s usually measured in millimeters. A large exit pupil is advantageous under low light conditions and at night because the larger the exit pupil, the brighter the image. For astronomical applications, the exit pupil of the telescope plus eyepiece should correspond with the amount of dilation of your eye's pupil after it is fully dark-adapted. This number will be between 5mm and 9mm. 9mm of dilation is the maximum amount for the human eye. Maximum dilation tends to decrease with age. By age 50, the exit pupil may be close to 5mm. An exit pupil larger than your dilation just wastes light from the objective, since the outside of the beam just falls on your iris and doesn't go into your eye.

To calculate the exit pupil, simply divide the aperture of the scope’s objective mirror or lens by the magnification of the eyepiece. Or divide the eyepiece focal length by the f/ratio of the scope. Example: an 8-inch (203.2mm) telescope with a focal length of 2032mm is used with a 20mm eyepiece. The exit pupil of this combination is 2mm.
2032/20 = 102x
203.2/102 = 2mm exit pupil
20/10 = 2mm exit pupil