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 Abu Dhabi Intl Airport flight would often be abbreviated as a YYZ to AUH flight. Toronto Island Airport (YTZ), Hamilton Airport (YHM), Toronto Kitchener-Waterloo Airport (YKF) are some additional IATA codes for airports related to your trip.