Mobile Substrate is saurik's extension for SpringBoard that allows 3rd party SpringBoard addons to run. Examples are the five-icon dock and WinterBoard. However, possibly one of the most useful is SBSettings that allows many tweaks to the SpringBoard, as well as enabling and disabling SpringBoard/Mobile Substrate extensions
And with all this extra code.........
As allways, even with PCs, extra code means crashing and Mobile Substrate can solve this too. Should the SpringBoard crash for any reason (which us quite common once let loose on Cydia as there are absolutely loads of SpringBoard extensions) Mobile Substrate will temporarily put the SpringBoard into Safe Mode with SpringBoard extensions such as WinterBoard diabled allowing you to continue using your device until it is convenient to respring