Plain English Meaning
It's the military's scheduling system for their special-use airspace. It keeps track of when each chunk of military airspace is hot or cold, and shares that information so civilian pilots and controllers can see whether it's safe and legal to fly through.
Full Definition
A U.S. military system used to schedule and track the activation of Special Use Airspace (SUA), including Military Operations Areas (MOAs), Restricted Areas, and Military Training Routes. MAMS provides the authoritative real-time status of military airspace and feeds that information to the FAA and to civilian tools that show pilots whether SUA is active, scheduled, or cold.
Context Anchor
Seen when planning a flight near special use airspace where military activity may affect whether a civilian aircraft can safely or legally pass through.