These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a London Heathrow Airport to Paris Charles de Gaulle Airport flight would often be abbreviated as a LHR to CDG flight. London Gatwick Airport (LGW), London Stansted Airport (STN), London City Airport (LCY), London Luton Airport (LTN), London Southend Airport (SEN) are other airports near this flight route and their unique identifiers/IATA codes.