Typically, the planets shift slightly eastward from night to night, moving slowly against the backdrop of stars. Sometimes, however, they change direction. For a few weeks/months, they will move west before turning back around and resuming their easterly course. This is retrograde motion. The planets do not really reverse direction around the sun, the effect is caused by the motion of Earth.
Mercury is a planet which rules transportation and communication issues. What happens when Mercury is retrograde? You miss appointments, your computer equipment crashes, and trades go bad.
The next Mercury retrograde period is June 26 – July 20.