Automation Testing without opening browser or any UI

I have been doing automation testing and it is working fine. It is working like open a browser, interact with elements and execute test cases based on given conditions.

I am using Selenium Webdriver, TestNG, Java and Maven for this.

Now from seniorI got request that we perform same test cases without open browser or interact with UI. How it is possible in selenium webdriver?

They said they will provide me API, It is possible to do same using API. If yes then kindly provide a small example so that I can know more about it and will implement then.