The lens closest to your eye is the eye lens . A field stop narrows the area through which the light passes, and it can serve a number of purposes. It may simply hold a lens that is smaller in diameter than the inside of the eyepiece barrel. Another purpose is to act as a baffle to minimize reflections on the interior of the eyepiece.
Most eyepieces have threads on the inside of the barrel at the end near the field lens.
The threads also allow the collar to be screwed into the barrel to hold the lenses in place. Never remove any of the lenses, since repositioning them correctly is a job best left to an optician, and will probably void any warranty on the eyepiece.