AstroQoA few times a year Mercury appears to move backwards in the sky. It's an optical effect: Earth and Mercury orbit the Sun at different speeds, so from here Mercury sometimes seems to slip back against the stars.
How long it lasts
About three weeks, three or four times a year, with a "shadow" period before and after.
What astrology links to it
Mercury rules communication, contracts, travel and technology. During a retrograde the classic advice is to review rather than start: reread messages, double-check details, revisit paused projects.
What it isn't
It isn't a reason to panic or freeze your life. It's an invitation to slow down and check before you sign.