The module documentation was originally made here Protected node external documentation. The documentation was remade here to allow anyone to update it.
The Protected Node module is used on websites where some pages need to be password protected. This is different from asking someone to create an account and often seen as much more user friendly, although in terms of protection it is quite similar.
If you've seen photo websites where you can go to an album but have to enter a password to see the photos, that's one example how this module can be used.
The module adds a field set in the node form to let authors enter a password.
The module has many options that are rather tricky to use so this documentation tries to explain what you can do with the module. When combining the module with others, it is quite powerful !