Plain English Meaning
It is the extra route-related details shown on an enroute chart, beyond just the airway lines themselves, that pilots need to fly that route correctly.
Full Definition
A category of supplemental data printed on instrument enroute charts that supports flight planning and navigation along published routes. It typically includes items such as changeover points, holding patterns charted along airways, mileage breakdowns, minimum enroute altitudes (MEA), minimum obstruction clearance altitudes (MOCA), minimum reception altitudes (MRA), maximum authorized altitudes (MAA), and similar route-specific notations.