Safari’s Reader mode is a way for users to peruse a webpage without distractions from ads, videos, sponsored content links, and other web elements you may not consider essential to the article you are reading.
In Safari 10 (the version that comes with macOS Sierra), Reader mode has to be enabled manually (View > Show Reader or press Shift-Command-R). With Safari 11 in macOS High Sierra, you can set the browser to open most pages in Reader mode. (Some pages, like the homepage of news sites, can’t be opened in Reader mode. But the individual articles can.)
Here’s how to set Reader mode in Safari 11 for macOS High Sierra. What this will do is open any pages that are Reader compatible in Reader mode when you visit a particular website. (This how-to uses the High Sierra public beta. It will be updated when beta updates and the final version are released.)