Google Labs is a part of Google where employees work on projects that aren’t necessarily ready for use in products like Gmail yet. In other words, they’re just experiments.
But Google allows you to use them if you’d like, and there are some interesting ones for Gmail, including adding custom keyboard shortcuts and a gadget which shows your Google Calendar.
To turn on labs features, head to Gmail settings, and then hit the “labs” tab at the top of the page. There, you’ll be able to search for a specific lab, and then turn each one on or off individually.