Authentication is a mechanism within a system that verifies the identity of the requestor as genuine; this mechanism has nothing to do with the assignment of rights or permissions. After a requestor has been authenticated, authorization can then be granted, providing access to system resources.
Authentication is required on three different occasions for on-premise installations of Microsoft SharePoint:
User authentication. A user is trying to access SharePoint resources
App authentication. An installed app is trying to access SharePoint resources
Server-to-server (S2S) authentication. Two-way resource access between servers (Exchange/Lync/SharePoint) in the enterprise
SharePoint does not provide any ...