This is so exciting! I just learned it this week via +Lars Fosdal on Google+. I love learning new stuff! Makes my day every time! You can extend your Gmail address so basically what this means is that you have your typical email address i.e. firstname.lastname@example.org. You can extend it by adding a +word to your username. The e-mail will still come to your regular inbox but it gives you a way to see how they acquired your e-mail address in the case of spam. Here are some examples:
While this isn't something that I think I would use everyday it is something that I would defiantly use when I reluctantly sign up for a newsletter that promises not to sell my info. Very exciting!