When you forward a message as attachment or you just attach saved message it will be sent as a file of type message/rfc822. Roundcube could display text content from inside it, also its attachments, but that was all. Now you can do more with it.
The attachment of type message/rfc822 can now be previewed the same as you can preview images or other formats supported by your web browser. Just click it.
The preview window will contain additional Reply, Reply-All, Forward and More actions buttons. Which means you can now for example respond directly to such a message or use it as a template for a new message.
As we already are in freeze for the next major stable release this is too late for this feature. It will have to wait for 1.3 and it will land in git-master as soon as we branch off 1.2.