A/B testing (sometimes called split testing) is a method of comparing two or more versions of a web page or app against each other to determine which one performs or convert better.

Use case:
You have a new service but can't decide the best way to market it on your site.

You'd create 2 pages and then using 3rd party tools or code - You'd show visitors variations of the page and track the results with analytics software to find the better choice.