For questions related to usage, errors and behaviour of navigation keys present in navigation bar or navigation buttons.
Back : The back button takes you back one screen from where you are. Also pressing and holding the back button will open all the list of opened apps or app switcher.
Start : Pressing the start button will take you to the start screen. Pressing and holding the start screen will activate one handed usage i.e) all the contents will be moved to the bottom of the screen.
Search : If you have Cortana enabled, pressing search button will open Cortana and upon pressing and holding search button you can speak to Cortana.
If you don't have Cortana enabled, pressing search button will open up bing search. Pressing and holding search button will open speech.
On phones without built-in buttons, you can reveal the navigation bar by putting a finger below the screen and swiping up. It will contain the three navigation soft keys.
There are some customizations available for navigation bar such as auto hide, swipe down gesture, colour change, etc...