There are many instances where a user fills some input in a form, edits that input, and then might attempt to leave the page that they're on. However, often, we'll want to secure the form in such a way that if someone navigates away or closes the browser tab, they should be prompted to confirm that they really want to leave the form with unsaved data.
Whenever these kinds of instances occur, you will see an alert appear on the top of your browser like this: