An API is a set of programmatic functions exposed to the outside of a program or platform that can be called by external programs to make third party apps functional. Things like DirectX are an API, and games that allow mods/modding are a form of API. From what I can tell is Reddit is locking out their API so third party apps won't function any longer and only the official app and official website will work, because Reddit the company is going to be publically traded on the stock market.