Plain English Meaning
It is the FAA's reference book that tells you everything you need to know about a specific airport — what its runways are like, who to talk to on the radio, what services are available, and any special procedures to be aware of.
Full Definition
An FAA publication containing detailed information about airports, seaplane bases, heliports, and related navigation facilities within a given region of the United States. It lists items such as runway dimensions, lighting, available services, communication frequencies, navigation aids, and special procedures or notes for each location. The A/FD has since been renamed the Chart Supplement, but the term A/FD is still widely used in and by pilots.