Here's a breakdown:

* Normal motion: Planets in our solar system generally appear to move eastward across the sky relative to the stars. This is due to Earth's own motion around the Sun.

* Retrograde motion: Occasionally, planets like Mars appear to slow down, stop, and then move westward for a period of time before resuming their normal eastward motion. This is an optical illusion caused by the relative positions and speeds of Earth and Mars as they orbit the Sun.

How it happens:

1. Earth's faster orbit: Earth orbits the Sun faster than Mars.

2. Overtaking: As Earth "overtakes" Mars in its orbit, Mars appears to slow down, stop, and then move backwards (westward) against the background stars.

3. Perspective: This retrograde motion is a matter of perspective. It's similar to seeing a car on a highway seem to move backwards when you pass it.

Important note: Retrograde motion is not a real change in the direction of a planet's motion. It's an apparent phenomenon caused by the Earth's perspective as it orbits the Sun.