Going on the name alone you’d have a hard time figuring out exactly what the functionality was and why you would want to preview your inbox?  But the feature is aimed at people who want to access their Gmail accounts through a slow connection.  Gmail can take quite a while to load fully, but that might not be something you have noticed if you only ever access it through a speedy connection. 

On a slow connection you could be sitting there for several minutes only to find out you didn’t have any new e-mails you wanted to read anyway.  So Google have come up with a solution.  While Gmail loads properly it will display a static image showing the ten most recent messages added to your Inbox.  Based on that information you can decided whether to continue letting Gmail load or use the time elsewhere. 

A very simple feature, but an effective one if you are relying on a slow connection.  I tried enabling it while accessing Gmail through my fast home connection and you don’t even have time to look at the preview image.  But it is something to keep turned on if you don’t know how good some connections you will be surfing with are.