Here's what DB says about them:
Ruhebereich
Für Fahrgäste, die sich mehr Ruhe und Entspannung wünschen, haben wir unsere Ruhezonen optimiert. Sie befinden sich entweder in Abteilen, den ICE Lounges (Abteile hinter dem Fahrzeugführer) oder in kompletten Wagen.
In diesem Bereich sind Handytelefonate, Klingeltöne, lautes Musikhören (auch via Kopfhörer) oder sonstige lärmende Tätigkeiten nicht erwünscht.
I couldn't find an equivalent page on the English site, so I'll do a rough translation myself:
Quiet zone
Our quiet zones are optimised for passengers who want more peace and quiet. They can be found in compartments, ICE lounges or in whole carriages. In these areas, phone calls, ringtones, loud music (including through headphones) and other loud activities are not welcome.
In principle, people shouldn't be making phone calls or having loud conversations in the Ruhebereich. It's not a silent zone and you shouldn't be surprised if passengers travelling together talk to each other, but it is a quiet zone and they should make an effort to keep their voices down while doing so.
If other passengers are being noisy (as opposed to just not being silent), you'd be well within your rights to point out where they are and ask them to adjust their volume/location. But I've not had to do this personally and couldn't tell you how it's likely to be received. I would note that I find the signage not especially clear (relatively small stickers in places you probably aren't paying attention to when trying to find seats, especially if doing so with small children in tow!). I can easily imagine that a lot of those not respecting the quiet zone simply don't realise they're in one at all.