Start here. This is the direct spoken answer to practice first.
Overview
Authentication answers who or what is calling; authorization answers whether that caller may do this operation.
Authentication verifies credentials or a token and produces an Authentication representing the caller. Authorization evaluates that authenticated identity, its authorities, the requested operation, and often the target resource to decide whether access is allowed. Spring Security can authorize at the HTTP request and method boundaries. A valid login does not imply permission to every endpoint or every record.