Most peerages are hereditary, meaning that they pass on from father to son, or to another heir.
A peerage passes from father to son, but sometimes a peer dies without a son to succeed him. In this case, that would be 5th Duke's other sons, if he had any. Lord George died during the 6th Duke's lifetime, but if he had survived him, he would have become the next duke. This man became the 7th Duke of Devonshire. But if Lord George's line had died out, then the dukedom could be traced back up to three more generations, all the way to the 1st Duke, and descend through the eldest of his other sons who had surviving legitimate male issue.