Rhythm Combo usually means guitar, keyboard, double-bass (or bass electric guitar),  and drums. It is the basic minimum combination for jazz, big band and popular music groups. When part of a larger group such as a Big Band, it is called the Rhythm Section.

A rhythm combo will often accompany a singer, or a solo instrumental player such as a trumpeter or saxophonist. If you add a clarinet, trumpet and trombone you get a Traditional Jazz band. If you add a lead guitar and a singer you get a rock group.