Fallback Mode is for when X11 can't start preventing the Graphical User Interface (GUI) from being displayed. Usually if this is the case if drivers are missing, not compatible, or corrupt. Video fallback mode then uses a generic video driver to display the GUI. This can usually be seen with higher end video cards but is not always the case.
I hope this cleared up any issues you might have had, please let me know if you have any other questions!
-Sam
