The direction of the star is reversed as indicated by the call. If dancers are in a right-hand star, on the call "Back by the Left," all drop hand holds, make a right face U-turn back (in some areas, dancers clap hands once while doing the U-Turn Back) and then form a left-hand star to move forward in the opposite direction.
This definition is from The Handbook of Modern Square Dancing by Jay King.
It is provided here for informational and educational purposes only.