This is the type of authentication request being made. 'password' is most common if using username and password from end user to grant access.

This is the client_id provided to the developer for accessing the api. Will match with the client_secret

This is the client_secret provided to the developer for accessing the api. Will match with the client_id

This is the username and logic for the user. This may look like a uid (user@domain) or could be something else if setup diffferent including a email address potentially.