http://msdn.microsoft.com/en-us/library/windows/desktop/hh848037(v=vs.85).aspx
http://www.technospot.net/blogs/windows-8-what-really-happens-in-automatic-maintenance/
Basically, just various optimizations, error checking and reporting, possibly updates. Automatic maintenance should only run when you are not using your computer, and should stop within 2 seconds of you starting to use it if it is running unless something important is going on (or something is being done that hasn't been done in a very long time, but is deemed necessary to be performed by the maintenance service). In these instances, there will be a performance impact since it will be occurring while you use the computer, but it will give you a notification and you will have the option to stop the maintenance.
When I'm using my computer the icon 'Maintenance in progress' is still there. Strange that it doesn't stop automatically then, should I just stop it manually?