APIs are a way for multiple applications to talk to each other. Most often this will be the toddle application running in your users browser talking with a back-end service running on a server.

Diagram of a browser and server talking via an API

API is short for Application Programming Interface but everyone just says API. API is an umbrella term for any kind kind of interface between two systems, In our case we are specifically referring to APIs exposed by a web server using HTTP as the underlying protocol.

If you want to learn more about what APIs are, we wrote an article about i on our blog

Setting up an API