These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a San Francisco Airport to Armidale Airport flight would often be abbreviated as a SFO to ARM flight. While you’re still figuring out your travel plans, it’s not a bad idea to learn the names and codes of other airports around these areas: San Jose Airport (SJC), Oakland Airport (OAK), Santa Rosa Sonoma County Airport (STS), Napa County Airport (APC), Concord Buchanan Fld Airport (CCR).