What does what? In simple terms, the tiny peephole narrows your focus so the target and foresight almost appear in focus together. The turrets on the rearsight move the eyepiece so your point of Aim and point of Impact coincide: when the rear aperture moves you instinctively move the barrel so the sights are aligned.

The foresight tunnel houses the foresight proper, shading it from light. The foresight itself is a stamped metal or Perspex disc. Perspex foresights have a chamfer around a central hole, in the tunnel the chamfer appears as a floating black ring.

Centre the foresight in the rearsight, then centre the target and release the trigger.

If you want accessories, start with extra inserts for the foresight, in a range of sizes - both metal and Perspex. These govern the gap around the target, too small an aperture can cause aiming errors.

An adjustable eyepiece is useful, so you can compensate for ambient light. Some just adjust the inner aperture. Others have colour filters to make the target stand out more; yellow is useful indoors under fluorescent light. Some have polarisers too, these are useful outdoors to tone down very bright sunlight. For indoor only shooting a simple adjustable iris is good enough.

You can get eyepiece that have an adjustable focus lens: these do the same job as glasses, but aren't permitted in some competitions. Personally I find glasses (or a lens attached to the sight) simpler and easier to keep clean.