Eye relief put in simple terms is the distance between your eyepiece lens and your viewing eye.
How much is good eye relief?
The most important consideration is that given the eye relief you should still be able to have a good field of view. The normal eye relief is 9 to 13 mm.
Why is it important for the people who wear eyeglasses?
For people who wear eyeglasses the eyeglasses can increase the distance between your eye and the eyepiece of the binoculars. Now if the distance is more then you will see a part of the image.
Most manufacturers provide the ability to have the eyecups came down for the people who wear eyeglasses but then that is generally not a good solution. You should buy binos which are built specifically for people who wear the eyeglasses. The eye relief for these binoculars is more than 14 mm.
If you wear thick eyeglasses then always go for binoculars which are 17mm or above. A lot of people use spare eyeglasses which are of thinner lenses while viewing through the binoculars. This way you do not need to buy the special binoculars and can buy almost any binoculars available in the market.
Tips: As a general rule have your eyecup down when viewing from the eyeglasses and if you are viewing through the naked eye then get these eyecups in up position.
Always make sure to check the eye relief specification on the binoculars. A lot of manufactures do not put this specification out. You can do a manual test to get this specification. This manual test will involve holding a piece of paper behind the binoculars and viewing the binoculars against a light source. You will see a image of the source as a bright spot on the paper. The distance at which the spot will appear the brightest is the eye relief distance.