PAP is a protocol that allows you to authenticate your login and password when logging into a server without using encryption procedures. It is obsolete, as it is used only for logging in to older servers running on UNIX.
The protocol is provided by two systems. The first system is the remote device sending the request. The second system is the data validation system that checks the request against a database.
PAP is almost never used in modern information systems because it is an insecure protocol due to the fact that the login and password are transmitted in plain text.