What do you “see” as the soldiers march to the front? Why do they joke about the rows of coffins?

The men are scared as they march to the front. Paul sees lines of them in columns marching to an almost certain death. Their faces are grim. They see over a hundred new coffins roadside on their way over. The men joke, but understand that the coffins are for them. Joking is the only way to stay sane when you are marching into oblivion.

Oh, it is not merely what they can see but what they can hear as well. They can hear the French and British artillery is strengthened. This, of course, is most frightening to them.