Safe Mode is enabled when you open a CrashAutoSave.toe file which is created when TouchDesigner crashes.
This mode opens the file in a static state where nothing will cook. This allows you to edit and navigate the file to investigate cause of the crash, without the file crashing again.
You can rename any file and put "CrashAutoSave" at the beginning of the name to make it open in safe-mode. For example: CrashAutoSave_myfilename.toe will open in safe mode.