First of all you need to understand what Safe Mode is. iPhone enters in Safe Mode to prevent a permanent crash cycle. It also gives the user a chance to remove an issue that causes malfunction without the need to perform a full restore of your device. In most cases, Safe Mode is a temporary glitch that can commonly be fixed by tapping Restart. That should exit you out of Safe Mode and solve the problem.
If the Restart doesn’t fix the SpringBoard crash, it might be caused by an incompatibility with one of the tweaks installed. Also check to make sure the tweak is compatible with the firmware you are running. If you suspect a certain tweak just uninstall that to exit Safe Mode. But since it’s hard to know for sure which is the tweak or package causing the error, open Cydia, select Manage and then Packages. Start by checking the info available for every installed tweak and find which is not compatible. Use Cydia to remove the problematic tweak.
For a fresh start you can remove all installed tweaks. Do that by uninstalling Mobile Substrate in Cydia. Once they have been removed, you can always reinstall your favorite tweaks, but this time be sure to check compatibility and safety. Download tweak packages from the likes of BigBoss, Cydia/Telesphoreo, ZodTTD/MacCiti and ModMyi.