A/B testing, also known as split testing or bucket testing is a method of collecting data about the performance of different versions (called A and B) of a single element, such as an email campaign, website, landing page, app interface etc. The goal is to identify changes that increase engagement or conversions.
A/B tests are commonly used in web design for conversion rate optimization. These tests are implemented using two variants that are identical except for one variation that might influence a user's behaviour.
For example, there may be two images on an eCommerce site each showing similar products but with different prices. For each picture, they're tested against another where instead of one product at a higher price there are two lower priced items to see which one has a greater conversion rate.
There are many tools available to perform A/B testing, but a good free tool is google optimize.