What does API testing mean?

When a new API is created by the developer and sent to the tester for testing, how should the new API tester begin with testing this API?

This seems to be a simple question, but after reading many articles about the API, I am completely confused.

Can someone please indicate the correct way to understand what an API is, why it is needed, what needs to be tested, and how can it be tested?

Note. I am completely new to the API and very poorly versed in the API and its concepts in detail

Thanks at Advance