Gmail Labs are experimental, pre-release features that users can enable to add key business functionality to their Inboxes.
When Gmail Labs are enabled, you can go to a Labs tab in your Gmail settings to turn on the Labs they want to use.
One of the most useful ones that we love is called "Preview Pane." This is great if you really miss the preview pane feature of traditional email clients like Outlook. Check out this article to learn how to use it: https://gmail.googleblog.com/2011/08/new-in-labs-preview-pane.html
Also, this LifeHacker article has some great suggestions of other Labs that could be useful: http://lifehacker.com/5721986/top-10-gmail-labs-you-should-enable