Safe Mode puts your phone in a diagnostic state (returned to default settings) so you can determine if a third-party app is causing your device to freeze, reset or run slow.
An alternate method is available if the device is unresponsive or won't power on.
Press and hold the Power button until the 'Shutdown' option appears then release.
Touch and hold Shutdown until the 'Restarting' prompt appears then release.
The process may take up to 30 seconds to complete.
Upon reboot, 'Safe mode' appears in the lower left of the unlock/home screen.