Karma Vasana means that a human being may die with sorrow leaving behind certain desires getting unfulfilled in one lifetime. Say for example a person is interested in getting name and fame as a musician or dancer or an actor etc. and has worked his or her lifetime to accomplish the same and dies without achieving it before his or her death. In the next birth this knowledge which is hidden in the soul gets manifested as a scent from past birth and he or she pursues this knowledge to accomplish the fame in the next human birth which otherwise could not have been accomplished in the past births. So, it is the desire of any form that tends the soul to get rebirth.