API testing ensures that the APIs that the software is using work as expected. The tester writes code that makes an API request to the server that provides the API, provides the required inputs, collects the output from the response, and matches the actual output with the expected output.
API testing primarily concerns the business logic of the software that’s exposing the API. It does not involve the look and feel, accessibility, or usability of the software. API testing can be automated to make it repeatable and reproducible each time they run.