A jamb saw is one of a type of power or hands saws - they're a specialist tool and used for undercutting the casings on doors, before the install of flooring takes place. They're intended to reduce labor - instead of removing the casing, fitting the hardwood flooring and then cutting and re-fitting the door - the jamb cutter can be used whilst the casing is still in place.
They're available in two types - either a circular power type saw (don't think along the lines of a circular saw though) or a hand held that uses a flat saw blade with the teeth being limited to the front edge. Of the two, the flat bladed variety gives the better finish - the power type is preferred due to its ability to speed up the process.
The five best undercut saws are featured below. They're all well rated, offer great value for money and are popular with other professionals and home improvement enthusiasts.