The meaning of OAuth redirect URIs

Because of my limited understanding about the technical term used in Facebook Developer,

I have been struggling hard to implement even the login provided by Facebook. 

SO I'm writing this for future reference:

Valid OAuth redirect URIs = the URL (or link) of where your Facebook login placed.

Meaning of OAuth redirect URIs

Eg. 

if your Facebook placed under the URL http://localhost/facebook/login/
then you can insert either http://localhost/ or http://localhost/facebook/login/ (exact URL) in the Valid OAuth redirect URIs input box.