The metaphor compares the sharp edges of dead prairie grass to the sharp edges on an anvil.
A metaphor is a comparison the author makes between two unlike things in order to help you understand something you don’t know by comparing it to something you do know. In this case, you have to understand what an anvil is.
An anvil is a tool that was historically used by blacksmiths. It is made of metal and was very heavy. It was shaped kind of like a triangular block, although the ends were flared. The edges were sharp. I have included a link so that you can look at a picture because it is easier to see the picture than for me to try to describe it! A prairie, on the other hand, was a large grass land. When grass dies in the summer it gets very hard and actually sharp. Basically, the author is saying that the grass became sharp when it died. The answer to your question is actually sort of in the next line after your quote.
The grass turns brittle and brown, and it cracks beneath your feet. There are green belts along the rivers and creeks, linear groves of hickory and pecan, willow and witch hazel.
When the grass dies, it becomes sharp, and if it is closer together it is as sharp as the edge of an anvil. When the writer makes this metaphor, it is with the assumption that the reader knows what an anvil is (and what a prairie is). Picture the prickly sensation of dead grass, exaggerated by the fact that prairie grass is somewhat sterner stuff. Then you have your almost dangerously sharp edged dead prairie grass.
Images like this help to establish a mood. Like the dead prairie grass and the myth, the Kiowa culture is dying out or in danger of dying out. Thus we end where we began—with death, at a cemetery.