The focus pseudo-class allows you to select the element that has focus, which can be useful for verifying that a specific JavaScript fired and an element has focus. This technique is preferred over using XPATH because it is easier to read, simple, and generally less brittle. The code snippet demonstrates how to use this pseudo-class in Ruby to check if no element has focus on page load and then sets the focus on the first input after sending a tab to the page, types "Hello WebDriver!" into it, and asserts that the text is present.