These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Blountville Tri Cities TN/VA Airport to Reykjavik Keflavik Intl Airport flight would often be abbreviated as a TRI to KEF flight. Reykjavik Domestic Airport (RKV) are other airports near this flight route and their unique identifiers/IATA codes.