Simply: Return to Original Speed
More: To resume to the original speed/tempo of the movement/piece especially after a deviation. It is a directon given by the conductor/musical piece.
Tempo means time (in italian). Tempo is usually indicated by beats per minute (BPM).
Usually in classical music this is specified only in terms of a general tempo, using Italian terms such as Presto (very fast), Allegro (fast) (see previous post) or Adagio (slow).