See the demos for instructions on how to create an OAuth app for linkedin, twitter, vimeo, facebook, github and google. When wrapping an API from a package, the author may want to include a default app to facilitate early and casual use and then provide a method for heavy or advanced users to supply their own app or key and secret.
appname: name of the application. This is not used for OAuth, but is used to make it easier to identify different applications.
key: consumer key, also sometimes called the client ID
secret: consumer secret, also sometimes called the client secret. Despite its name, this does not necessarily need to be protected like a password, i.e. the user still has to authenticate themselves and grant the app permission to access resources on their behalf. For example, see Google's docs for OAuth2 for installed applications.
redirect_uri: The URL that user will be redirected to after authorisation is complete. You should generally leave this as the default unless you're using a non-standard auth flow (like with shiny).