Normally, nautical GPSs indicate this value with a "r" (right) and an "l" (left) depending on whether we are off course to the right or left of the line that joins the two points (departure and destination) and that represents the ideal route.
This parameter is normally expressed in miles (or decimals of this measure) and indicates the minimum distance to take us back to the line of the ideal route. But of course we will rarely need to return immediately to the line of the ideal route, but following this indication we will correct our route, so as to bring us slowly on the line of the ideal route or perhaps following the indication of the given STR.