Cag
In short
CAG is the IATA airport code for Charles de Gaulle Airport, the largest international airport in France.
In plain words
It is the main airport serving Paris. Most international flights land here, and it uses several different terminals to manage millions of passengers every year.
A simple example
A traveller flying from London to Paris might land at CAG and take the train into the city centre.
Why it matters
It acts as a massive hub for European travel and air freight. Because of its size, it is a vital link for global transport networks.
Easy to mix up
Do not confuse it with CDG, which is the more common three-letter code used for the same airport.