In simple terms, ARINC 429 is the "language" and "wiring" standard that allows different pieces of equipment on an aircraft (like the GPS, the autopilot, and the cockpit displays) to talk to one another. 

Messages are sent unidirectionally over the bus at 12.5kbps (“low speed”) or 100kbps ("high speed”). 

ARINC 429 uses a point-to-point architecture with a “one talker, many listeners” setup. In a typical setup, one transmitter can send data to up to 20 receivers on a single wire pair.