Plain English Meaning
It is the FAA's master list of all the special airspace areas in the country. It tells you where each one is, how high it goes, when it is in use, and who is in charge of it.
Full Definition
FAA Order JO 7400.8 is the official FAA publication that lists every piece of Special Use Airspace (SUA) in the United States. For each area, it gives the legal name and number, the lateral boundaries, the vertical limits (floor and ceiling), the times of use, the controlling agency, and the using agency. It is the authoritative reference pilots and controllers consult to know exactly where SUA exists and when it is active.
Context Anchor
You may see this order referenced when studying how to plan an IFR route, review charted airspace, or determine whether a crosses a , , warning, , military operations, or firing area.