I don’t understand APIs that well in regards to real life usage. Can anybody help?

So I’m doing a tutorial and I’ve writtenAPI calls in Java and using Postman to test them.

I understand how they work in this context. But I don’t get what this would look like in a real life application?

What would it look like in real life? Is it maybe when someone’s entering in some info on a site they are actually doing a post request for example online shopping and wanting to search for say books.

but the developer have dressed it up all nice so they don’t have to worry about the nitty gritty?