API as a term has become synonymous with client server interactions over the internet. As such APIs use the HTTP protocol to communicate. HTTP stands for HyperText Transfer Protocol, and interestingly, when you think of API calls (things like GET, POST and PUT), they are just directly using commands built into HTTP. These commands are not something that are defined in some API standard outside of the protocol. They are actually part of the protocol itself. This article gives a nice summary of HTTP at a high level.
If you want to dive a bit deeper into the detail of this, you can follow along with this tutorial. If you are testing APIs, it is probably a good idea to grasp at a basic level how HTTP works. Honestly, if you are testing anything client/server based on the web it is probably a good idea to have a basic understanding of this stuff. I must confess that I really hadn’t read much on HTTP before. I’m thankful for this challenge that pushed me to do this!