I think you meen 'blimps' the American term for derigidable airships. The things used on d-Day were more accuratly called Barrage balloons, I do not know is the americans had a different word for them. If you put 'Barrage balloon' into Wikipedia and you should get an explanation.
In short, a large hydrogen filled balloon fixed to the ground by some thich steel cables with other steel cables hanging from it . this would stop a plane from flying near it as the cables would slice into a wing or knock the plane into other cables, the ground or a balloon filled with imflamable gas.
When if a dive bomber could dive between the balloons it could not pull up out of the dive to climb steep enough to avoid the cables.
Both the British and the Germans used these balloons to protect land targets as well as at sea. Special cable cutting aircraft were developed by both sides to cut the cables by having sharp hardened steel blades in front of the wings and propellers, or as well having small explosive charges along the blades to explode on contact with the cables.
The balloons sometimes escaped and caused some problems by damaging electricity and telephone cables, fences and buildings as well as interferance or Radar.