DG = disorganized. Over stacked hexes are automatically disorganized. Stacks can also become disorganized as a result of enemy bombardment. Any units under a DG have all their stats halved, attack, defence, movement.
There is one step in the Allied phase where all DG counters, both sides, are removed from any stack that isn’t over stacked. This happens after the Allies move, so they get affected by DG for moving. The next action is Allied Bombardment followed directly by German Bombardment.