German forces attacked Allied forces (primarily from the United States and Great Britain) in the Ardennes region.
Further details:
The Battle of the Bulge, also known as the Battle of the Ardennes, was an offensive attempt by German forces to push back the front line of Allied forces that had encroached into previously German-controlled territory. Winston Churchill is the one who first referred to the "bulge" that the Germans had pushed through into Allied forces.
The Germans were initially quite successful in their offensive. However, Allied forces managed to hold their ground after the initial "bulge" of the Germans. Eventually, the Allies pushed Germany's forces back. And the effort overall cost Germany too much. The Allies suffered about 75,000 casualties, but Germany lost 120,000 men and much military equipment and supplies. Germany was exhausting its strength. The overall result of the Battle of the Bulge was that it crippled and demoralized Germany's effort to continue against Allied advances.