When you visit a website, a small window may pop up and obscures part of the page. This is called a pop-up. Pop-ups are usually advertisements that appear in front of or behind the web browser window, in another tab, or as part of the current active window. Many people find them annoying because they interfere with their ability to view the underlying content on a webpage. Some pop-ups are so large that they cover most of the screen and force you to close them before you can continue using the site.
Pop-ups are created using JavaScript, a programming language that allows for interactive content on websites. When you load a page that contains JavaScript code specifying how and when to display a pop-up, your web browser runs that code. That's why some ad blockers work by preventing JavaScript from running on websites altogether—if there's no code to run, there can be no pop-up windows generated.
Some sites use what's called an overlay popup—a smaller window that appears on top of the main pagecontent and doesn't obscure it as much as traditional pop-ups do. These types ofpop-ups can be used for things like email sign-up forms or social media buttonsand generally aren't considered as annoying as traditional advertisingpop-ups because they don't make it difficult to view the main content onthe page.