Open Market is a set of policy decisions and a software and services framework that will allow interoperability of various formats and DRM schemes that are currently splintering the market. That splintering locks users into a single store and format, and is putting a stranglehold on widespread adoption of movie sales online. Multiple sources have indicated that the studios are putting their weight behind the initiative to avoid the fate of the music industry and as a last ditch effort to stop or slow non-DRM movie sales.
The Coral Consortium, one previous attempt at addressing interoperability issues, seems to have stalled. It remains to be seen whether the Open Market initiative will be any more successful.