User experience testing has several different names - user testing, usability testing, UX testing. The objective is to find out how users interact with your site and why they take certain actions. For example why they stay on your site or leave to visit a competitor, why they abandon a shopping cart or simply why they don't buy at all.
Usability testing will tell you why something happens on your site as opposed to what happens which can be tracked by analytics software and is what your conversion goals and sales funnels are designed to do.
One of the less publicised advantages of using a universal shopping cart like Shopify is that everyone reaps the benefit of user experience testing because everyone is using the same checkout process. Doing usability testing on your own can get very expensive.