Karma means “action” — more precisely, intentional action and its trace. Every intentional act leaves a tendency behind, shaping perception and future action, compounding across a life and (in traditions that hold it) across lives.
Popular usage flattens karma into reward-and-punishment, but the traditions are subtler. Karma is not a moral accountant but a feedback loop: what you practice, you become; what you become, you perceive; what you perceive, you then act from.
In Jainism, karma is nearly physical — subtle matter clinging to the soul. In Buddhism, karma without a self is often explained by analogy — the flame of one candle lighting another, one moment conditioning the next without any substance passing between.