I always see them in pictures I but can't figure out their purpose. Was it the allies who dropped them there as cover for the troops or did the Nazi's put them there to make it easier to aim their guns?
They were called Czech hedgehogs, and they were a form of anti-tank obstacle. They were put in place by the Germans to help defend against Allied armour. If a tank tries to drive over a Czech hedgehog, it stands a good chance of riding up on it and getting stuck. They therefore function a bit like another type of anti-tank obstacle called dragon's teeth, but are easier to construct and put in place than the permanent dragon's teeth. They also provide less cover for enemy infantry than dragon's teeth. They are often pretty big things, as seen in this picture.
I found an image of a Soviet IS-2 tank attempting to drive over concrete Czech hedgehog-like obstacles. The IS-2 seems to be doing a pretty good job of it, but one can see how many tanks could get slowed down or stuck on these devices.
