What are the warning areas?

Warning area refers to airspace of defined dimensions, which extend from three nautical miles (3NM) outwards from the coast of the U.S that may contain activity which can be hazardous to non-participating aircrafts.

Warning areas are one of the six special use airspaces. The other special use airspaces are: prohibited areas, Military operation areas, Alert areas, restricted areas and Controlled firing areas
A warning is set in these areas to warn non-participating pilots of possible danger. Contacting the controlling agency of that area before entering a warning area is advised but not mandatory.