It’s a Central Vacuum Inlet
The large circular opening is designed to accept the hose of a central vacuum system.
Instead of carrying a traditional vacuum cleaner from room to room, central-vacuum systems use tubing installed inside the walls of the house. You connect a special vacuum hose to an inlet like the one shown in the photo, and the central power unit provides the suction.
Central vacuum systems commonly use dedicated wall inlets, and manufacturers such as NuTone and BEAM offer replacement inlet valves and related components.
