Usually set at the factory, the rack stop keeps you from cranking the objective lenses too far down (damaging something).  If you are using a very thin slide, you may find that you can't get the high power objective lens close enough to the slide to focus.  Here you can either adjust the rack stop or place a thin glass slide under your origina [..]

A safety feature consisting of a mechanical stop, typically adjustable, which prevents the objective lens from hitting the microscope stage. Top