Adblock is a browser plugin that stops third-party ads from displaying. This hurts small sites that run on ad revenue. But just like code can block ads, code can also detect when ads are blocked and put up something else instead.
What should you show visitors that block your ad? Something worse than an ad. Suck it, adblocker.
These are real and you can use them. Here's the github repo with the banner files and the code to install.