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.