Pop-under refers to a type of online advertisement that appears in a separate window or tab behind the current webpage being viewed. Unlike pop-up ads, which appear in front of the webpage, pop-under ads are only visible once the user closes or minimizes the current window. This form of advertising is often used to promote products or services and can be triggered by clicking on a link or by simply visiting a website. While some may find pop-under ads intrusive, they can also be an effective way for businesses to reach a wider audience and increase brand awareness.