Retrograde means “backwards movement.”
The Earth has an orbit around the Sun. So does Mercury. Because it is the closest planet to the Sun, it’s orbit is much shorter/smaller compared to the Earth’s orbit.
About three (sometimes four) times each year, Mercury, goes whizzing by Earth.
To an observer standing on Mars, Earth appears to be moving ‘backward’. Mercury is surging ahead and Earth is left behind. This is ‘retrograde motion’.
Consider this idea to understand retrograde motion further.
When riding in a motor boat, the engines are in ‘forward’ position zooming you around. When you put the engines in reverse to ‘back up’ the boat, you can observe turbulence beneath the surface of water as the propellers switch direction. This is ‘retrograde’ motion. All the turbulence, frothing, bubbling and boiling as the water (which was flowing nice and smooth in one direction) suddenly switches to reverse motion.
Take Mercury and Earth in the same analogy: Mercury is the speedboat. Earth is the water in which it floats. As Mercury ‘passes’ Earth due to its shorter orbit around the Sun, this “turbulence” energetically is created between the two planets for a brief (3 week) period. Mercury passes Earth as it zooms around on it’s shorter orbit, creates invisible ‘waves’ of energy and this ‘energy’ is translated into ‘retrograde motion’.
Even though we can’t see the energy and turbulence as a faster moving planet in orbit affects a slower moving planet in its orbit, the ‘turbulence’ is still the same. Just because you can’t see it, doesn’t mean it isn’t there.
Astrologers, from 5,000 years ago, noted that when a planet passed us–or we passed another slower moving planet (from Mars outward–all orbits are longer around the Sun than the Earth’s orbit), they noted that certain events seem to occur. Not to everyone, but certainly a great portion of humankind is effected.
As the turbulence subsides from the retrograde period and the ‘water’ becomes smooth again, Mercury is then said to be ‘direct’. It takes Mercury three weeks to approach Earth in a parallel orbit (side by side like a team of horses) and as it nudges past Earth with it’s planet body, and finally moves ahead. When direct, all the Mercury Retrograde ‘symptoms’ cease and many breathe a sigh of relief.