Airport identifications

[n502pd]

I have asked several professionals about this, but have still not understood what is meant. An aexmple is at Mckinney National Airport, Mckinney Tx, just north of Dallas Tx.
Other airports around the country also have this desigination. On app 'flightradar24", the airports that have a blue teardrop location symbol, when I select one, there is a three letter ID prior to the actual airport id I am used to seeing(like KTKI, or just TKI), in this case, QQT/KTKI. There is an active tower here, and at the others that have the three letter attribute id's. In this app, flights leaving or arriving to KTKI, show as departing from QQT, or destinating at QQT, and not at (K)TKI. Why?

My question is,what do there three letters signify about the associated airport? Equipment on site, Passenger usage, type of runway, tower equipage and performance?

Thanks for enlightening an old guy!!!