The "Rhythm & Blues" term was created to replace the designation "race music," which until then was the stardard catch-all phase used in reference to most music made by blacks at the time. After the "race music" term was deemed offensive, Billboard began using the Rhythm & Blues name that Wexler created.
The meaning behind the name is this: the "rhythm" part comes from the music's typical dependance upon four-beat measures or bars and employ a backbeat (beats two and four accented in each measure). And the "blues" portion came from the lyrics and melodies of the songs, which were often sad, or 'blue' during the music's emergence in the World War II era. Over time the name was shortened to R&B as a matter of convenience.
By the 1970s, rhythm and blues was also being used as a blanket term to describe soul and funk as well. And today, the term can be used to loosely define most sung African-American urban music, even though soul and funk can be placed in categories of thier own.