Text in quoted italics is directly from http://www.mojonation.net/whatitis.html the rest is a summary
What is Mojo Nation?
"Mojo Nation is a data service distributed over the Internet where users can store and search for any data that others have supplied. It utilizes the growing global market for unused disk space, bandwidth, and CPU cycles. Resources are bought and sold in this market using "Mojo," a currency that pays owners of these idle resources and efficiently puts them to use for everyone."
That's a wide ranging statement, so it may be useful to break it up.
"Mojo Nation is a data service distributed over the Internet where users can store and search for any data that others have supplied."
Mojo Nation is a service with a lot in common with freenet. Rather than serving a piece of data(1) from a single point, it uses specially designed algorithms to distribute that data across the net in such a way as to make access as efficient as possible. If thousands of people in the UK start requesting a particular piece of data, it makes sense to store that data in the UK, rather than keep it in Outer Mongolia (common sense really, but many "global" sites still live in the US - E2 is an example of this)
Users are allowed to both give and take from Mojo Nation. If you have something to contribute, you can store it in the Mojo Net. If you need a particular piece of data, use a Mojo search engine and retrieve it - You don't need to know where it is, just that it exists.
"It utilizes the growing global market for unused disk space, bandwidth, and CPU cycles."
While Seti may not have proved the existence of alien transmissions reaching Earth, it has proved that there is a vast amount of unused resources on millions of computers around the world. Mojo Nation treats this prevously freely available resource as a transferrable commodity - just like money.
Data in the Mojo Net is stored on every participants hard drive, much like Napster. This data is encrypted in the same way as freenet, as are the requests for that data.
"Resources are bought and sold in this market using "Mojo," a currency that pays owners of these idle resources and efficiently puts them to use for everyone."
This is the bit of Mojo Nation that sets it aside from Freenet or Gnutella. The designers of Mojo Nation have decided to clearly define the value of the disk space and bandwidth that you are "renting" to the service. If you own a powerful PC and broadband network connection that is idle for 70% of the day, you can put those valuable resources to good use. Your PC could earn you Mojo while you sleep or work or study.
Resources or data are paid for by a user's Mojo. Something valuable will cost more, a small piece will of course cost less. If you need the latest tutorials on a new programming technique, you would search for it (or maybe link from the author's web site), retrieve it (confirming your "purchase") and pay with a little Mojo.
So, to summarise. Mojo Nation will essentially look like Napster - allowing you to search for data, retrieve it and pay for it. You'll earn Mojo by renting/selling the idle resources available to your PC, and use that Mojo to pay for retrieving data across the Mojo Net.
(1) I hesitate to use any term other than "data" when describing Mojo Nation. I think that the problem and possible downfall of Napster is that it concentrates solely on MP3s. Data could be literally anything digital.