USB debugging mode is required to access development level of Android Phone, the primary function of this mode is to establish a connection between phone and pc with Android SDK (Software Development Kit) Tools. Android application developers develop applications and games on computer with these tools and then transfer these apps to device using debugging mode.
As we discussed that android debugging mode creates a connection between phone and computer to access and alter phone’s deeper level of setting. It also gives you freedom of control over the Android device. In debugging mode using SDK Tools you can access your phone directly from your PC.
To enable USB debugging mode in KitKat and Jelly Bean (4.2 and later)
“Settings > About Phone” and press Build number 7 times. Your developer options will be visible) and a short pop-up message will appear “You’re now a developer”.
You are done.
Now go back to Settings > Developer Options > USB debugging Mode to enable/disable this option.
To enable USB debugging mode in Android 4.1 or 4.0
Settings > Developer options and enable it
To enable USB debugging mode in Android 2.3 Gingerbread or earlier
Settings > Applications > Development > USB debugging
If you have any question regarding USB debugging mode, please feel free to ask in comment, I will try to give best answer.