This is one measure -- expressed in millimeters -- of the amount of light that reaches your shooting eye.
Exit pupil also describes the image that's projected to a particular point in space. Your eyes must be positioned at that exact spot in order for you to see the full field-of-view with maximum resolution. When holding the binoculars at arm's length, it's the small, circular image that's visible in each eyepiece lens. When your eyes are in the proper viewing position, the round shafts of light combine to create a fully detailed sight picture.
By the way, the average adult eye dilates to a maximum of 7mm in extreme low light conditions. For binoculars to deliver their full light gathering capability, the exit pupil must be equal to, or greater than the diameter of your pupil at any given moment.
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