Karma means "volitional action." A thought, word or deed conditioned by desire, hate, passion and illusion create karma. When the effects of karma reach across lifetimes, karma brings about rebirth.
Karma is the principle of causality in moral experience Cf. "As fruit of good deeds done, promising men much profit in new births for works of faith" (Bhagvad Gita)
The parable of Karma
In order to catch spider monkeys, hunters in South America simply walk through the jungle and drop heavy containers on the ground. These containers have very a narrow top and a wider bottom. Inside the containers the hunters drop a special kind of nut which is particularly attractive to the monkeys. Sometime later, the spider monkeys come down from the tops of the trees, smell the nut, but the tops of the containers are so narrow they have a tight squeeze to get their hands inside. Once they grab the nut at the bottom, their fist is too large to remove if through the opening. And the container is too heavy for them to carry.
So instead of letting go of the nut, the monkeys just sit there until the hunters come back, pick them up, and throw them in a bag.
The spider monkeys are not prepared to let go of a small nut in order to gain their freedom.