The ICAO airport code as used for navigation. They're different from the IATA 3 letter codes that most passengers know. US airports start with K, European with E (e.g. EF for Europe-France).
Often for large airports the code is very similar to the IATA code for passengers, e.g. KJFK for JFK and KSFO for San Francisco. But not always, as in EGLL for London Heathrow which is LHR in passenger codes.
EF is Finland actually. The first letter is region code, and Europe is divided into multiple regions. Northern Europe is E, Southern Europe is mostly L (so France is LF).
Wikipedia has a nice map showing the different boundarys on it's ICAO airport codes page.
> Often for large airports the code is very similar to the IATA code for passengers
This is mostly only the case in the United States, since it has a single-letter K prefix, so they can append the 3-letter IATA code to make a valid 4-letter ICAO code. In the rest of the world the second letter of the ICAO code is used to designate countries, so the 3-letter IATA code cannot be appended.
ICAO-registered U.S. airports in the contiguous 48 states start with a K. Alaskan and Hawaiian airports start with a P and other U.S. territories get other different initial letters depending on their region (e.g. TJ for Puerto Rico or NS for American Samoa).
