API stands for application programming interface. It’s a set of rules that govern how one program can talk to another.
In the Pokémon TCG, the API is how your computer talks to the Pokémon TCG Online application. When you connect to the game, your computer sends a list of commands to the Pokémon TCG Online application, and the application sends back the results.
This system allows you to do all sorts of nifty things. You can create programs that keep track of your collection, build decks, and play games against other people. You can also use the API to create more sophisticated tools, like deck builders and simulation programs.
One of the coolest things about the Pokémon TCG API is that it’s open source. This means that anyone can look at the code that makes it work. If you’re a programmer and you want to create your own Pokémon TCG tools, the API is a great place to start.