Go to  (or) Settings > About phone and tap twelve times quickly on Build number. 

A warning will appear to let you know the number of remaining taps before accessing this option. 

A new menu  will appear in the settings list, corresponding to options for developers. 

 This menu gives you access for instance to USB debugging and various options, specific for developpers. It is not recommanded to access this mode if you don't know the possible consequences of activating some features that can damage your device.