From Halopedia, the Halo wiki
HBOrg Special is an achievement in Halo: The Master Chief Collection. It is awarded for activating the Club Errera Easter egg in the level New Alexandria. This achievement can also be done with Play Us a Sad Song, Claude, which the first step of this Easter egg is required to do.
The name of the achievement is a play on 'HBO Special' - referring to TV specials produced by the company HBO; and Halo.bungie.org - commonly abbreviated 'HBO' (or as 'HBOrg' for this achievement). The achievement takes place in Club Errera, named after the creator of halo.bungie.org, Claude Errera.