|It looks to me as if the message is saying that (very sensibly), the system won't play DVDs on the main screen. That would be because it could be a distraction to the driver. Presumably you can connect other screens which would be located where only the rear-seat passengers could see them.|
As you don't actually tell us the model of the device it's difficult to be more precise. It's difficult to think that this isn't mentioned in the manual.