What does Brutus mean by "Ambition's debt it paid" ?

Read the excerpt from part 2 of “to build a fire.” to build the fire he had been forced to remove his mittens, and the fingers had quickly gone numb. his pace of four miles an hour had kept his heart pumping blood to the surface of his body and to all the extremities. but the instant he stopped, the action of the pump eased down. which words from the excerpt provide context clues for the underlined word?