Authentication is the process whereby we confirm that someone or something is who or what they claim to be, or more specifically some entity should be treated as being allowed to act as the someone or something.
To put it another way, authentication is a process in which we make sure someone is who they claim to be. In web services, this is usually done by something claiming they are a particular entity and then providing some piece of information to confirm (make authentic) their claim.
In TiddlyWeb authentication is done through a combination of a challenger and a credentials extractor. The challenger provides an initial interface through which a user can make and confirm their claim. A credentials extractor is then used to verify that claim in subsequent requests.