This allows third-party publishers to publish games and supplements using the popular D&D ruleset. They are doing things slightly differently this time round (they first made the system Open license with the release of 3rd Edition). Interested parties can pay $5000 now for all the infromation, updates and the ability to start publishing material on 1st August 2008 (2 months after the official release) or they can wait until 6th June 2008 to download the SRD for free and start publishing material under the OGL on 1st January 2009 - five months later than the companies who pay for the license.
Full information is here : 4E SRD and OGL (Wizards.com)
Now some of the early companies publishign material for 3E made a killing and grew into influential industry movers and shakers (Green Ronin and Mongoose being the main two that spring to mind). Whether 4E has the same effect is something open to debate but I think if backed with good quality products several companies could do worse than paying the $5k now for early access and to be in the first wave of third-party products.
I also wait to see if many of these 'first wave' are just 4E rehashes of old 3.0 & 3.5 books (the resurgance of the Slayers Gudies and Quintessential books anyone?).