Inbox zero is something that, I believe, Google introduced when they created gmail. I may be wrong but that is pretty much when I first learned of this term. Being a strong advocate of a clean inbox as well as gmail myself, I found it time to help those that might wish to achieve such a goal.
Inbox Zero as reported by whatis.techtarget.com rigorous approach to email management aimed at keeping the inbox empty -- or almost empty -- at all times.
With gmail this is something that can be achieved quite easily, especially if you take advantage of the Priority Inbox and Archive features.
For those using Outlook, and I know many of my clients (still) do, this achievement is much harder to accomplish but it doesn't have to be!
Ideally you are already using a gmail account for your mail but if you like to open your mail using Outlook you can still utilize the archive folder. It is located as a sub-folder of gmail, named "all mail" so by simply moving your messages to this folder will be utilizing the archive feature. If you aren't using a gmail account then you can always create an ARCHIVE folder and then use this. As well, you should then be utilizing the built in auto-archive features of outlook .
The biggest thing I witness when helping a client with computer issues, is the simple fact that their inbox has hundreds, and in some cases, thousands of messages and as a result they find themselves having troubles either finding something or manipulating aspects within an email message such as opening or viewing an attachment.
By getting that inbox cleaned out, you will find yourself having much less problems when dealing with your email. Plus, who doesn't love a clean house/inbox!