What is the "Automatic Maintenance"?

According to official msdn site

http://msdn.microsoft.com/en-us/library/windows/desktop/hh848037(v=vs.85).aspx

Automatic maintenance regularly checks for solutions to problems (via Windows Error Report), runs the .NET Optimization Service and defrags all hard disks automatically -- all of this happens while the PC is on idle, of course.

If your computer is in Sleep mode, while on AC Power, it will be resumed and the maintenance activity will be carried out using full system resources to execute the task as fast as it can. After completion of the task, the system will be returned to Sleep mode. You will however, have to check the Allow scheduled maintenance to wake up my computer, if it is plugged in at the scheduled time option.

When Automatic Maintenance is being run, you will see its indication in the taskbar icon.

You can check out more details here http://www.thewindowsclub.com/automatic-maintenance-windows-8

This feature appears to optimize the performance of Windows 8 according to THIS POST:

Automatic Maintenance is designed to address these problems with maintenance activity in Windows:
- Deadline scheduling
- Resource utilization conflicts
- Energy efficiency
- Transparency to the user

According to THIS POST it will

perform maintenance tasks like software updates, security scans and system diagnostics