This is the distance (in mm) the binoculars can be held away from the eye that still allows the user to see the entire full field of view as designed. Binocular users who wear eyeglasses for near- or far-sightedness can remove their glasses while observing clearly through the binoculars because the binoculars can fully correct for these eye defects. However, the users with astigmatism may need to wear their glasses in order to maintain sharp imaging through the binoculars. In this case a longer eye relief will become advantageous for these users, who cannot get as close to the eyepiece.
As a rough guide, an eye relief of 10-15mm should fit most users if this function becomes necessary. However, you may not need to look for this specification because most binocular models allow the eyecup to be either folded or twisted/pushed down to enable eyeglass wearers to enjoy an improved field of view.