A common scenario in a website (or web application) is opening up a new browser (or tab) on the click of a button. With these multiple browsers, windows can be automated using Selenium test automation.
Once the Selenium WebDriver instance is instantiated, a unique alphanumeric id is assigned to the window. This is called window handle and is used to identify browser windows. Since the id is unique, it is used by the Selenium WebDriver to switch between different windows (or tabs).