AfD is a far right populist party in the EU's biggest economic powerhouse country, whose explicit goals are to leave the EU (they probably can't due to the German constitution), exit the eurozone, withdraw from the Paris climate deal, leave NATO, and cozy up with Russia.

It's not hard to imagine what kind of damage they could do to the EU if they took power in Germany and started working with Hungary to block EU legislation, veto sanctions, defund programs, etc.