From Halopedia, the Halo wiki
The mirror was a plot device in the I Love Bees alternate reality game. Specifically, it was a time-space connection between sections of the fragmented "smart" AI Melissa, which had been divided by a Slipspace anomaly that had affected the UNSC Apocalypso.
The anomaly had fragmented Melissa into Durga, the Operator, and the Sleeping Princess; the former was sent to the year 2552, while the latter two ended up in 2004. The three fragments were aware of the mirror's presence, but only the Sleeping Princess and the viral Seeker recognized it for what it was. The term, coined by the Sleeping Princess, likely originated from the AI fragments' inability to send a data query through it; the query was simply reflected back to its sender. Despite this, audio clips of Durga's spying filtered in through the mirror; the Operator was later able to use the mirror to manipulate Durga into spying on Herzog, and the Seeker used it to send the previously-mentioned audio clips back to Durga.