What is is.
The login name used with protected pages.
What it says.
It tells you the login name when URLs on the server are accessed that are protected. This is what happens: the browser sends the request as usual. The server finds access is restricted, and tells the browser so. This is when your browser pops up a window requesting a login name and a password. Upon completition, the browser sends the same request again, but this time with the authentication information consisting of login name and passwort. If the information is correct, the server returns the URL and writes the login name to the server log.
What may cause trouble.
This works only for protected pages; for other pages the field will be "-" (not set). Since group logins might be used, the login name often will not identify individuals.