While complexity of web tech means there's usually some hidden nuance, that description on MDN seems pretty clear: a non-interactive (other than for navigation) iframe, ie an <a> element that's a preview.
Scroll to the bottom and you'll see that it must be enabled by the user, is experimental, and not listed in any HTML specification.
Looks to me like it's intended as a link with preview, and part of the idea is that user agents could do a sort of animated transition morphing the preview into the content of the destination page.