How APIs Work

"REST API Concepts and Examples" by jelledTV

The Application Programming Interface (API) is a key concept in all software. The API is the way that programs are able to interact. APIs are designed to follow specific protocols, and they reveal functions that can be invoked. If you have an app that can play an audio file, then another piece of code can invoke that audio playing capability via the audio app's API. 

The key thing to keep in mind when thinking about APIs is the term "interface"--this is why the API exists. Users and other software each require their own interfaces, and each has a unique set of qualities that make the interface better or worse.

To get an idea of how all of this works and goes together, watch the video above and read the required texts about what makes a good API design. The additional resources on this topic cover the full span of knowledge level, so use those to fill in or extend your understanding as you feel comfortable.