“Engagement is an even more nebulous term. We want to track that the user is giving some amount of active attention to the site. To approximate this, we register a user as active when the user scrolls, clicks or types on the page. Media playback also provides a small engagement increase.”
What it looks like. You can try it yourself, just by copying and pasting chrome://site-engagement/ into your Chrome browser bar. Here is a screenshot from Dan:
Why we care. It is not clear if Google may or may not use this for Google Search ranking signals or not. Many would assume they do but Google might deny it. Either way, you can imagine that having this data in aggregate can be super useful to understand which sites have the most positive engagement from its user base.