These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Christchurch Airport to Manila Ninoy Aquino Intl Airport flight would often be abbreviated as a CHC to MNL flight. Cebu City Mactan Intl Airport (CEB), Angeles City Clark Intl Airport (CRK), Panglao Bohol–Panglao Intl Airport (TAG), Davao City Francisco Bangoy Airport (DVO), Boracay Godofredo P. Ramos Airport (MPH) are some additional IATA codes for airports related to your trip.