These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Nagpur Dr. Babasaheb Ambedkar Intl Airport to Dallas/Fort Worth Airport flight would often be abbreviated as a NAG to DFW flight. Dallas Love Field Airport (DAL), Waco Airport (ACT), Tyler Pounds Field Airport (TYR), Dallas Addison Airport (ADS) are some additional IATA codes for airports related to your trip.