What retrograde actually is
No planet moves backwards. The issue is perspective.
Planets do not reverse. Retrograde is when a planet APPEARS to move backwards as seen from Earth. It is entirely a perspective effect.
Why it looks like it moves back
Think of a motorway: as you overtake a slower car, for a moment it looks like it is sliding backwards. That is exactly what happens as Earth overtakes a slower planet.
Figure
This diagram is drawn live in the app, on your own chart.
How astrology reads it
In astrology a retrograde is read as a period of review: finishing what was left half done rather than starting something new, reading the contract again, returning to an old subject. This is why Mercury retrograde gets associated with communication.
Mercury goes retrograde three times a year for about three weeks each. That is roughly a fifth of the year — if it were a period when everything breaks, we would have noticed.
What does a planet do during retrograde?
The loop it traces in the sky is an illusion of viewpoint.