This document discusses the importance of user testing to understand why people behave in certain ways on websites and products, such as bouncing from pages or choosing certain options in A/B tests. It provides tips for running simple user tests with only 3-5 participants to get fast results, such as testing in coffee shops or using online tools. A list of 13 things that can be tested is also included, such as website overviews, shopping carts, and mobile experiences. The overall message is that user testing is crucial to understand user behaviors and catch usability issues.