This level of authentication allows clients to make requests on behalf of users who have authorized the client application. API endpoints requiring this level of authentication will require signed requests which include a valid user access token.
Read more about making user-level requests here.
Clients wishing to issue API requests without requiring users to authorize their application can do so by taking advantage of application-level authentication. This method still requires signed requests, but the signed request does not contain an access token. Note that not all API endpoints support application-level authentication; refer to the individual endpoint documentation to check what level of authentication is required.
Read more about making application-level requests here.