Plain English Meaning
It is the agreed-upon data format that tells flight management computers how to store and interpret navigation information, so that every approach, departure, and route looks the same to the avionics no matter who built them.
Full Definition
ARINC 424 is the international industry standard that specifies the format and content of navigation data used by airborne navigation systems, including FMS and GPS units. It defines how waypoints, airways, airports, runways, terminal procedures (SIDs, STARs, approaches), and path-terminator are encoded so that any compliant avionics can read and fly the data consistently.