The next sentence says: "Plus, Gmail will start suggesting when to unsubscribe from newsletters or offers you no longer care about."

And the following screenshot: https://storage.googleapis.com/gweb-uniblog-publish-prod/ima...

Shows: "Try notifications for only your most important emails: Turn on / No thanks".

So in context, the message is not that "high-priority notifications" mean louder ringtones but that (if you accept) Gmail will try to notify you only when an incoming email is important. That hopefully means fewer interruptions from newsletters and other bacn.