It means the charger probably is not providing enough juice to charge your iPad at a good rate. It probably still is charging, but slowly. (Look at your battery percentage for increase, and if you see it declining rather than increasing, stop using that charger. If it's increasing but slowly, then you decide whether that's fast enough for you, or you go with the OEM wall wart or another charger instead.)
From my experience, third-party chargers, even namebrand ones, can develop this "not supported" problem when there are firmware upgrades. (I don't know why, but it's happened to me before with iPod/iPhone chargers that worked perfectly fine before and then suddenly gave me that message. Even on iDevices that charged perfectly before, I've had that happen with such chargers.)
What has always worked for me: the OEM Apple chargers and the Griffin PowerBlock.