Authentication provides a way for you to tell the Brass service that you are whom you claim to be; when Brass confirms this information, you are issued something [a token] that allows you to identify yourself for future calls to the API for a time frame.
The token issued to you after successfully authenticating is used to authorise (see Authorization) future requests to the API.
The only way to get authenticated is via use of the email and password for the user profile to be authenticated (see Acquiring a JWT).