Three or four times a year, Mercury appears to move backwards for about three weeks. It is an optical effect of Earth and Mercury passing each other in their orbits. Nothing in the solar system breaks.
Popular culture has turned these weeks into a season of blame. Lost emails, delayed trains and misunderstandings happen every week of the year; during a retrograde people simply notice them more.
The traditional meaning is useful and modest: Mercury's themes — speech, paperwork, travel, learning, trade — turn toward review. It is a fine time to edit, repair, reconnect with old contacts and finish what was left half-done.
Signing a contract during a retrograde is not forbidden. Read it twice, ask the extra question, and confirm details in writing. That is good practice at any time.
The sign and house Mercury retrogrades through show where the review is most personal. Retrograde in your second house may concern budgets; in your seventh, a conversation with a partner that needs a second round.
Watch the shadow periods before and after, when Mercury covers the same degrees. Issues that appear in the first pass often return during the retrograde and settle after it. Patience, not fear, is the lesson.