Eye relief is the range your eye can be from the scope to be in focus. It is not necessarily the distance from the scope. For example a really nice scope might have a 3" eye relief but it starts 1" from the scope. So your eye has to be no less than 1" and no more than 4" from the scope.
Usually you can get quite close to the scope, like say 1/2", and I even have one scope (semi-permanent loan to my room mate for his M14) that has an almost 6" eye relief (only 1.5x though.)
I have owned some really cheap NCStar scopes that had 1/2" or less eye relief, there was one super precise point your eye had to be at for the scope to work at all, was horrible.
Usually the higher the magnification, the worse the eye relief. So for say a 3-9x scope it is usually really easy to use at 3x but you need to take your time and be more precise at 9x because of the lower eye relief.
Last edited by LUTNIT; March 3rd, 2009 at 15:42..