The 2" Male to 1.25" Female Drop-In Eyepiece Reducer 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.

In some cases, an Extension Tube may be necessary to retain path length and properly focus the scope.