These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Detroit Metropolitan Wayne County Airport to Washington, D.C. Dulles Intl Airport flight would often be abbreviated as a DTW to IAD flight. Baltimore/Washington Airport (BWI), Washington, D.C. Reagan-National Airport (DCA) are other airports near this flight route and their unique identifiers/IATA codes.