These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Phnom Penh Pochentong Airport to Bangkok Don Mueang Intl Airport flight would often be abbreviated as a PNH to DMK flight. Siem Reap Airport (REP), Krong Preah Sihanouk Sihanoukville Airport (KOS), Siem Reap–Angkor Intl Airport (SAI), Krong Khemara Phoumin Dara Sakor Intl Airport (DSY) are some additional IATA codes for airports related to your trip.