This shortcut decreases the indent in a cell by one step each time it is used.
On Windows, you can sometimes use Ctrl+Alt+Tab to indent and Ctrl+Alt+Shift+Tab to un-indent. However, The application switcher in Windows 7 seems to conflict with these shortcuts.
On the Mac, Ctrl+M and Cmd+Shift+Tab also work. Cmd+M should work, but it conflicts with the the Finder minimize window shortcut (see http://apple.stackexchange.com/questions/73886/disable-command-m-to-mini…)