The International Civil Aviation Organisation (ICAO) has established a well-known and widely used aerodrome designation system; more widely known as ICAO Airport Codes. It comprises of 4 letters, and is simple to decode.
The first letter usually corresponds to the wider geographical area the country of the aerodrome belongs to;
Depending on the area, what follows after the first letter is either the aerodrome designator itself, or the country code. For example, in the case of the United States, the aerodrome designator falls directly after the area; KJFK for the JFK airport in New York, or KMEM for Memphis. In most of Europe, the second letter designated the country code; EG for the UK, LF for France, ED for Germany. Then, the aerodrome designator will follow; EGLL for Heathrow, EGCC for Manchester, EDDH for Hamburg.
In the aviation industry, and the Network, we use these ICAO country codes for flight plans. If you are unsure of the airport that the ICAO code corresponds to, you can simply look it up on Google, or say the full code phonetically (Echo, Golf, Lima, Lima, for EGLL). Over time, as a controller you will find you learn many of these codes to memory by being exposed to them.