A stem raiser fits into the steering column in place of your present stem, and secures with a wedge bolt in the normal manner. The upper part of the stem raiser sticks out above the steerer, and is bulged out so that you can insert your original stem. The bottom of the original stem winds up a bit above the top of the headset, so you gain several inches of height.
This size fits older U.S. made bikes with one-piece cranks, such as Schwinn, and most BMX bikes. Actual outside diameter of the riser is 13/16" (21.15) mm.
Note, you may need to install longer brake and/or shift cables.
You'll need a long 6mm allen wrench to tighten the bolt in the stem raiser.