The Messenger God of ancient myth, Mercury is the speaker, writer, chatterer, jokester. That means, from our perspective on Earth, he appears to be moving backwards.
Communications get a little mixed.
People are more likely to misunderstand each other. Mercury doesn't cause this to happen, but rather reflects this type of activity in the world. If you sign papers during a Mercury Retrograde cycle, you may find yourself reversing, re-doing, or regretting the deal later. Things change when Mercury goes direct. You see things differently than you did during the retrograde cycle.