The discussion centers around player piano rolls, with a specific reference to a user's search for MIDI files that correspond to these rolls. The user found a substantial collection of MIDI files available on Archive.org, highlighting the transition from traditional player piano technology to modern digital formats. They also mention their own project involving a historic player piano that they are modifying to retain its original functionalities while enabling MIDI control. Additionally, a unique aspect of player piano technology is noted, where vacuum is used to detect holes in the roll, emphasizing the mechanical ingenuity of these instruments.