These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Toronto Pearson Intl Airport to Surabaya Juanda Airport flight would often be abbreviated as a YYZ to SUB flight. Vancouver Intl Airport (YVR), Montreal Pierre Elliott Trudeau Intl Airport (YUL), Calgary Airport (YYC), Edmonton Airport (YEG), Ottawa Airport (YOW) are some additional IATA codes for airports related to your trip.