SAFE MODE allows you to turn on the device with third-party apps disabled. Then you can easily remove all apps that maybe causing a conflict or a software problem.

First way to enter the device into Safe mode:

Press the Power off button. In the "Power off" menu tab and hold Power off option until you see "Reboot to safe mode" menu.

Second way to enter the device into Safe mode:

Turn off the phone. Turn on the phone and when you see the Xperia logo press and hold the Volume down button. Hold it until the device enter into Safe mode.

To go out from Service mode, simply turn the device off and on.