The fix is good the explanation is not:
mDNS comes in handy in an all Linux or a Linux/OSX/iOS network since it provides lan side DNS services when none are present.
This notification is informing you that mDNS (Avahi) has been disabled. It's only used for a small number of applications that only work on the local network, it won't adversely affect your internet connection or DNS. The most well known use for mDNS is sharing music with Rhythmbox (or iTunes) over your LAN. It's an Apple technology, but it's largely been ignored in favour of uPNP or DLNA.
Can't ping to another machine by hostname? You can with mDNS:
Your other linux machine can't find your samba share? You can access it directly with mDNS:
It's the mechanism by which all apple products and now all Linux devices can find and communicate with each other and it's hardly a technology that's been ignored as the above quote suggested.