The Gmail development team is permanently working on improving Google's email tool and also adding new features to make it even more useful. But more often than not, they need help from us, Gmail users, to test those new tools and give them feedback about their functionality and performance. This is why they decided to add a new tab to the Gmail Settings menu, called Labs.
If you enter this tab, you'll be presented with a list of new features developed for Gmail but which haven't been implemented yet. These new tools need to be thoroughly tested before becoming part of Gmail's set of features, so now you're offered the possibility to test them live in your own Gmail account and report bugs, should you find any.
These experimental new features include handy options like having stars in different colors, browsing your mailbox with mouse gestures or using random quotations in your signatures, among others. But being a gamer, the one I liked most by far is being able to play Snake in Gmail. It's as easy as enabling the "Old Snakey" feature in the Labs tab and then pressing the '&' key in Gmail's main page.
Bear in mind though that you also need to have keyboard shortcuts enabled (which you can do in the General tab, also in Settings) in order to make this feature work.