Retrograde
A planet is retrograde when it appears to move backwards through the zodiac from Earth, an effect of the two orbits passing rather than any change in the planet's motion.
Every planet except the Sun and the Moon does it. The apparent reversal happens when Earth overtakes a slower outer planet, or is overtaken by a faster inner one, and it is entirely predictable: our ephemeris tables mark every retrograde day.
Traditional astrology reads a retrograde planet as turned inward, reviewing rather than initiating, and treats the stations, the days it seems to stand still, as the most charged part of the cycle.
Frequently asked
Do planets really move backwards?
No. It is a line-of-sight effect between two moving orbits, which is why it can be calculated centuries ahead.
Is Mercury retrograde responsible for things going wrong?
That is folklore rather than classical doctrine. We treat it as a review period and nothing more.