except for localhost it must be over SSL
RedirectURL is where you send people after the oAuth dialog to complete the oAuth flow and obtain/use the accesss token (depending on oAuth flow type)
In my implict auth example the redirect URI is https://barrycarlyon.github.io/twitch_misc/authentication/implicit_auth/ For example
https://barrycarlyon.github.io/twitch_misc/authentication/implicit_auth/