The 2" Male to 1.25" Female Drop-In Eyepiece Reducer with Compression Ring allows for 2" focusers or eyepiece holders to use more common 1.25" eyepieces and accessories. Just push it in where the 2" accessory would go, and the rear (where light exits) has a 1.25" inner diameter barrel with a set screw that pushes against a brass compression ring. A compression ring provides a more secure fitting, and prevents any marring.
In some cases, an Extension Tube may be necessary to retain path length and properly focus the scope.