This is for things like clicking ads in the New York Times. Safari will open, and then at the top left of the screen there will be a "< NY Times" indicator. It's below the clock and above the aA icon in the URL bar.

Not just Safari. Most any two apps that support passing things from one app to the next (e.g. via Share Sheet, or a deep link, or etc.), the next app has a back to first app.

OK I see it now when I click on a link in the mail app. I've never seen it before because when I open a link with safari in the gmail app it opens a safari view inside gmail with a Done button at the top.