Plain English Meaning

It means the aircraft uses GPS to know where it is, and then flies a path between waypoints in space — not a path tied to a ground radio station. When you see GPS/RNAV on a chart, it tells you the procedure is meant to be flown that way.

Full Definition

A method of navigation in which the aircraft determines its position using signals from Global Positioning System satellites and uses that position to fly along any chosen course between defined waypoints, rather than being restricted to flying directly to or from ground-based navigation stations. On instrument approach charts, the label GPS/RNAV indicates that the procedure is designed to be flown using GPS as the source of position information for area navigation.