FEMA is the Federal Emergency Management Agency, the government body that steps in to coordinate the federal response when a disaster overwhelms local and state resources — hurricanes, floods, wildfires, and the like.
It shows up in trucking because disaster response depends heavily on freight: water, supplies, generators, and equipment have to be moved fast into affected areas. These “FEMA loads” are often urgent, high-priority freight, closer to a hot load than routine work.