Click "show details" and you can see when a message was sent in the sender's time zone as well as what time it is for them now:
Message headers always include the time sent and often include time zone info too. We use that information to show you these icons. If the time zone isn't included for a given message, this Labs feature won't display anything. Try it out and tell us what you think.
In all seriousness, we want to give a big thank you to all of you who use Gmail every day, to those who've been around since the beginning, to those who were using an AJAX app before the term AJAX was popular, to those who started chatting right in your email and then video chatting a couple years later, to those who changed your theme on day one, and to those who have turned on some of the 43 experimental Gmail Labs features (and put up with the occasional bugs they introduce)...we couldn't have gotten here without you. Thanks.