What's Degenerative Myelopathie In The GSD!?

I have heard of this while doing research on the German Shepherd Dog, but I don't really know what it is. How do these dogs even get this!? It sounds like a very horrible and painful disease to me!! How old do or can they be to get it!? How can I prevent my GSD ( I am getting soon) from getting this. Can it kill the dog!? Please answer with as much information as you can!!