The Airportle rules are quite similar to the standard Wordle rules. You have six chances to correctly estimate a three-letter airport identification code (LHR for London Heathrow, CDG for Paris Charles de Gaulle, and so on). The square will become green if you correctly guess a letter and it's in the right spot in the airport code. The letter will turn orange if it is in the code but not in the proper spot, and it will remain grey if it is not.
The airport's code must be listed on the IATA's (International Air Transport Association) official airport list. There are exactly 17,576 potential combinations, while it's not clear how many IATA codes there are in total. Which makes Airportle pretty challenging, especially if you don't care anything about airports.