This is the "Change File Selection" screen in CrashPlan. This is the screen that appears after clicking the Change... button on the main "Backup" tab.
Here you'll find a standard tree-style listing of your hard drives and other storage devices (like flash drives or other USB attached storage) that you can choose to have backed up to whatever destinations you've chosen.
Note: Mapped drives can not be backed up from. You can read more about why on CrashPlan's site here.
You can continually dig down through your drives and folders, selecting individual files to be backed up if you wish. A folder or drive can have either a checkmark, indicating that all other folders and files within are included, or a solid blue selection, indicating that some of the folders and/or files within are not included.
Clicking the Show hidden files checkbox will do just that, allowing hidden files to be selected or unselected in the list above.
The Cancel button will close the "Change File Selection" screen without saving your changes. The Save button will close this window, applying whatever changes you made.