It's the distance between your eye and the rear lens of the scope. A proper eye relief allows the shooter a full view through the device without impediment, (like the "cresent moon" shadow from the scope housing). Rifle scope generally have between 2 and 5" of eye relief.
Spotting scopes...I'm usually right on top of mine, regardless of the distance to target. Rubber eye pieces come in handy for that.
If you wear glasses it'll move your eye back away from the eyepiece, so look for a longer eye relief. Something to think about if you don't wanna remove your eye protection every time you use the scope.
The main purpose of eye relief is to keep shooters from getting little half-moon cuts over the eye when a scoped rifle recoils. Of course any good scope installer or intelligent shooter would make sure eye relief was sufficient based on the recoil of the rifle.
My cut didn't hurt much, but it sure bled a lot. The shooter was not intelligent at all. I could blame the stupid guy who put on the scope, but... You know the rest.
Adjusted the trigger on a 270 cal. First shot after the adjustment, I had a cut eye from my glasses. It wasn't an eye relief problem, just wasn't ready for the gun to fire. talk about a surprise trigger break:biglaugh: :biglaugh: :biglaugh:
If you wear glasses then more eye relief is better as Scooter 45 said but it also matters if you are using the spotting scope for target or field use in scanning for game. If you shoot target and require a bit of time to get into the correct position (such as prone) then having a long eye relief spotting scope means you don't have to get out of position to look through the scope. If you are shooting from prone on a hunt in really flat country the same applies but there is less movement to spook game. If you are scanning from behind cover then the scope will probably be on a tripod and long eye relief is less important.