A transformer for low-voltage wired door chimes. It converts 120-volt currents to a lower voltage so door chime will operate.
What’s a door chime transformer? It’s a device that connects to your household wiring and also to your wired doorbell. Electricity goes into the transformer at 110-120 volts, and goes through the transformer into the doorbell at a much lower voltage, usually 16-24 volts.