What is an ISP looking glass

Many ISP's provide something they call a looking glass.

a mirror; usually a ladies' dressing mirror

Source: Google

This is not what an ISP got. This is something that makes you look into their internal network from the outside.

A Looking Glass is a piece of software running as a CGI on an ISP's webserver that allows external users to get a look at routing and network behavior within their network. The software is usually a Perl script that resides in the CGI bin of a web server. The script executes shell commands, accesses a remote router, performes either a ping, trace, or one of several show commands allowing a view of the IP and BGP route tables. The CGI then returns the information as a web page.

Looking Glasses are most commonly used for verifying routing between providers, and for verifying that routes are propagating correctly across the Internet.

Source: InetDaemon