A NOTAM (Notice to Airmen) is an operational notice about temporary changes in airspace — new restrictions, closures, hazards or events. It is how the aviation system quickly distributes information that has not yet made it into permanent charts and documents.
For a drone pilot, checking NOTAMs is a mandatory part of flight preparation: a temporary restriction can override the picture shown by static geographical zone maps. A spot where flying was allowed yesterday may be closed today because of a rescue operation or a public event.