USB Debugging Mode
USB Debugging Mode is a mode that can be enabled in Android after connecting the device directly to a computer with a USB cable. The primary function of this mode is to facilitate a connection between an Android device and a computer with Android SDK (software
To enable this mode go into settings and click about device 5 times to show Developer options then open up Developer options and make sure the USB Debugging option is checked.
VERY IMPORTANT If you are rooting any Android Device with 5.1.1 make sure OEM Unlock is checked prior to performing root.