Adblockers do exactly what you'd think they do: block advertisements from loading on webpages.
They also block pop-up windows, pop-under windows, and trackers from following you around the web. If you've ever done a web search for something and seen it advertised back to you on a whole different website days later, you've experienced tracking cookies. (Annie once did a search for Disney and was seeing Disney ads for weeks.)
In addition to increasing your privacy and seeing less targeted advertisements, you're also passively protecting your computer from malware.
AdBlock is a free and open-source adblocker that's been around since 2009. It works with Chrome, Firefox, Edge, and Safari.
If you use Internet Explorer, Opera, or Yandex Browser, Adblock Plus will work for you. While it is also free and open-source, Adblock Plus allows "Acceptable Ads" by default. That setting can be turned off.