When a planet goes retrograde, it appears to move backwards through the skies. Of course, this is just an optical illusion, as no planet ever reverses its orbit.
In astrology, Jupiter is the fifth planet in our solar system. When it is retrograde through a natal chart or a transit report, it means that the energies of this planet are somewhat restricted, and will be interpreted slightly different than if it was in its usual direct motion. Jupiter retrograde can actually help to give you focus, and it is probably one of the easiest retrograde transits with which to deal.