It stands for rhythm and blues. It is a style of music.
View page
You are probably referring to "blues" music, or rhythm and
blues. The song "Summertime" is very "bluesy" sounding.
View page
Bill Withers' best selling album was Lean On Me and some others
were Ain't No Sunshine and Use Me. He got the Grammy in 1999 for
Best Rhythm and blues song for Ain't No Sunshine.