Retweeting is a way of forwarding a tweet from a Twitter user you're following, giving credit where it's due to the original tweeter. Most Twitter applications have a built-in feature that will allow you to RT something within two clicks. If characters are available, you can also add any additional comment you may have.

Example:
Original Tweet: @username123 I love Drupal!

If you want to RT it and spread the Drupal love, here's how you would retweet it.
Retweet: RT @username123 I love Drupal!