What Is My Browser
Assuming you're referring to the tool located at https://www.whatismybrowser.com/, it is a web browser detection tool. It works by looking at the user agent string that is sent by your web browser when it requests a web page.
The user agent string is a piece of text that contains information about your web browser and operating system. It also usually includes the version number of your browser. The What Is My Browser tool analyses this string and produces a report about your web browser and operating system.
This can be useful if you're trying to troubleshoot a problem with your web browser, or if you're trying to find out if a particular feature is supported by your browser. It can also be useful for web developers who want to ensure that their site works correctly in a range of different browsers.