What is retrograde motion?

It is the apparent backward movement of a body, overtaking a car that you are passing on the highway, it appears that the other car is moving backwards (relative to you).

In Astronomy this is most noticeable in the apparent motion of the planet Mars through the sky. As the Earth overtakes Mars the position of the red planet form one night to another among the stars slows and loops backward and then returns to it's normal direction (this takes several weeks or months).