The soft stuff inside of a bone is called marrow. Many people cook with marrow from beef bones and consider it a delicacy.
Dogs love to gnaw on bones, trying to get at the marrow inside, and many soups and other recipes call for marrow, which is tender and fatty. Human beings, of course, have marrow inside their bones as well, and certain diseases can be treated by marrow transplants. From this biological kind of marrow comes a figurative meaning of marrow — the essence of something. You might say, "This sentence represents the marrow of the novel."