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Chris,
The light that exits the eyepiece forms an "Exit Pupil".
The diameter of this exit pupil is the aperture of the telescope divided by the magnification. For,say, a 200mm scope with x50 magnification the pupil will be 200/50 = 4mm diameter.
This same exit pupil is being formed across the whole angular field of view and at a specific distance they all "cross" behind the eyepiece. This distance is the "eye relief" of the eyepiece.
When the eye is positioned at this point you can see the whole field of view without vignetting, "kidney bean" or shadowing. The eye relief distance can vary with the design of the eyepiece, hence some eyepieces are more suited to observers who wear glasses.
Hope this helps.
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