The planets move in their orbits so, to the observer on Earth, the planets change position compared to the background stars.  Normally, over time, they move west to east, which is called prograde motion.  However, they will regularly reverse direction and move east to west compared to the background stars, before turning back to west to east motion.  The reversing direction east to west motion is called retrograde motion.

The Ptolemaic model was geocentric, with the Earth at the center of the universe and the planets, Moon and Sun orbiting around the Earth.  To allow for retrograde motion the model used a series of circles.  The main orbit was called a deferent and the planet was located on a smaller orbit called an epicycle.  The full model is displayed below:

 The Ptolemaic model placed a stationary Earth at the center of the universe.  Since the Earth did not move in the model, it assumed that retrograde motion was real instead of an optical illusion created by Earth’s motion.  This meant that the model’s planetary motions were wrong.  The result was that the model could not accurately predict the location of the planets in the sky, because the planets’ motion was incorrectly described.

 The Copernican model placed the Sun at the center of the universe and the Earth and planets in orbit around the Sun.  Retrograde motion is an optical illusion caused by our moving point of view.  The outer planets have larger orbits than Earth, so the Earth overtakes, causing the planet to appear to move backward.  The inner planets have smaller orbits than Earth, so when they overtake us, they appear to move backwards.

 The simplest explanation for a phenomenon is usually the most accurate one.  The Copernican model did not need the complex motions required in the Ptolemaic model, consequently it was much simpler and so an improvement.

However, the Copernican model still made the incorrect assumption that planets moved in circles.  The difference between the predicted circular planetary orbits and the real elliptical planetary orbits meant that the Copernican model could not predict the position of the planets in the sky, any more accurately than the Ptolemaic model.

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Sources:

Bennett et.al., Cosmic Perspective 4^th ed.

Nature of the Universe – A brief History of Astronomy

http://www.physics.hku.hk/~nature/CD/regular_e/lectures/chap04.html

Nick Strobel’s Astronomy Notes:

Plato's Homework Problem:  http://www.astronomynotes.com/history/s3.htm

Renaissance: http://www.astronomynotes.com/history/s4.htm