Rhythm section

Rhythm section refers to the musicians whose primary jobs in a jazz or popular music band or ensemble is to establish the rhythm of a song or musical piece, often repeated riffs or ostinatos. It may also refer to the instruments of those musicians.

a non musician would call the rhythm section the background music. But their importance is much greater then that

In theory, any instrument or instruments can provide a steady rhythm (see Jimmy Giuffre's clarinet-valve trombone-guitar trio of the late 1950s, for example), and "rhythm" instruments often take featured solos, especially in jazz.

Most commonly, however, are a drum set, and a bass guitar or sometimes double bass. A rhythm guitar is a common feature, a keyboard and sometimes a piano instead, auxiliary percussion and/or other instruments may be used.

Rhythm sections for contempery popular music is not the only instruments. Usualy there is a Lead Guitar or/and Singer to play/sing the melody.

In some rare cases, the lower octaves of a piano, organ, or electronic keyboard may substitute for bass guitar. One of the best known examples of this was keyboardist Ray Manzarek of The Doors. Some jazz bands use tuba or other low-pitched instruments in place of the more common double bass. These methods are only used if a bass player is not avalible because they dont make the same effects as a bass guitar