Twitter search callback URL (confused)


Hi, just wanna ask about the twitter search Callback URL, in the app setting page (top right > my applications >“your app”)

What is the callback URL used for? ( I have a Callback.php file in my application )

Also, im testing my application locally and thus do not have a domain. If the callback URL does not to point to my application, tweets cannot be loaded into my widget after i have successfully been authenticated?

Thx in advance.

For more info on what im implementing: