The User flow report in Google Analytics help us understand how users move through different types of content on a website.

This is one of those report that exist in two different places. Once under Audience where it focuses on your audience and their Nationality as a break down. When the standard Dimension is chosen you get to see where different nationalitites land and whether or not the act differently on the site.
Under the main Behavior
For both of these views however, you can chose different Dimensions as your starting point and I suggest you play around with it.
When do I use the User Flow Report
Well, first and foremost I use it to analyze where people are dropping off and if there seems to be any navigational or informational needs they have when doing so.
Especially, I try to compare different usage amongst converters and non-converters using Segments. What I try to study is how they differ in terms of what content they consume before deciding to convert. If I find a pattern, I might decide to add or remove content from the landing page.
Together with tools such as Hotjar and survey data, I can get a pretty good idea of whether or not my pages are driving traffic to the goal I want it to.
If I have paid traffic coming to my website I might analyze whether or not that traffic moves around differently than other traffic based on the Ad Content, the Campaign or the time period the people reached my website.