Scale focusing is the procedure whereby the photographer estimates (or measures) the distance to the subject, and sets the camera's focusing ring or lever to the correct distance. Many simple cameras have no focus aid other than a focus scale, and a scale is provided even on cameras with advanced focus aids; SLR and rangefinder cameras, and on view cameras with a ground-glass screen. Guess focusing is a slang term for scale focusing.
As more advanced focusing methods (particularly rangefinders) became prevalent, scale focusing was only seen on moderately-priced cameras (i.e. those more expensive than fixed focus box cameras, but cheaper than rangefinder or reflex models). These simple cameras could of course be used with a hand-held rangefinder, or one mounted in the accessory shoe.
Scale focusing is likely to be unreliable at wide aperture, because the shallow depth of field gives little margin for error; however, most cameras sold with only scale focus would not have been equipped with a wide-aperture lens.