Safe mode prevents device from entering into crash cycle or gives a chance to restore device without causing a permanent damage. Jay Freeman's(@saurik), founder of Cydia, safe mode feature recovers devices from problems caused by jailbreaking tweaks.
In 2013 before Christmas iOS 7 jailbreak came along with a problem that Mobile Substrate feature hadn't been updated to support iOS7. That's why many of your favorite apps and tweaks would not be working well but now a good news for jailbreakers that Saurik has released an update for Substrate Safe mode to supports for iOS7 and 64-bit A7 devices like iPhone 5s, iPad air.
Description of Safe Mode in Cydia:
"Safe mode is a feature of Mobile Substrate that prevents your device from entering a permanent crash cycle by giving you a chance to uninstall malfunctioning extensions without a complete device restore."
Saurik tweeted "As of version 0.9.4000 this package is even safer on iOS7(and on ARM64)."
So whether or not you have had any issue, it is recommended to update to new Substrate Safe Mode via changes tab in Cydia, which recovers reboot loop problem.
