Most people who see "API" today only think "it's a web app I send a request to, and I pass some arguments and set some headers, then check some settings from the returned headers, then parse some returned data."
But "API" means "Application Programming Interface". It was originally for application programs, which were... programs with user interfaces! It comes from the 1940's originally, and wasn't referred to for much else until 1990. APIs have existed for over 80 years. Books and papers have been published on the subject that are older than many of the people reading this text right now.
What might've those older APIs been like? What were they working with? What was their purpose? How did those programmers solve their problems? How might that be relevant to you?
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