The term Karma (action, activity) has a wide number of meanings in Hindu philosophy and religion. Firstly, Karma can refer to any action, be it physical, mental, moral, etc. Secondly, it can be understood as any form of mental activity - the intentions underlying actions. Thirdly, it can be understood as the 'accumulation' of all actions during one's life to date. A central premise of the doctrine of Karma is that all actions have moral consequences and that 'good' actions are rewarded and 'bad' actions are punished. 'Good' actions are those which are performed in accordance with one's dharma. Moreover, Karma is both action and the 'fruits' of action. The concept of Karma is related to the doctrine of rebirth. However, in the Vedic period, Karma was primarily related to the correct performance of ritual. According to some scholars, it was the Jains who widened the doctrine of Karma to include all actions, and the Buddha who shifted the goalposts further to include intentions.Aog
Roughly speaking karma is built up for both good and bad acts. The balance of an entities karma will determine what their next life will be. Bad karma will result in being reborn as lower creatures. Good karma as higher, with more opportunity for improving karma further. The final goal to be to leave the whole rebirth cycle (samsara) behind completely. Although I've never quite understood why this is the goal.
A much more simple idea of karma exists in many peoples life as the idea that the world is self balancing. That good things happen to good people and bad things happen to bad people. Although a nice thought, I believe this to be demonstratably incorrect.
Another idea is that there is no law of conservation of wealth. (Wealth=well being, happiness, etc). That is, unlike energy, wealth can be created and destroyed. It is possible for everything to become more wealthy. We are all aspects of the universe. In this view doing 'ill' to other people is doing ill to another part of ourselves. It is bad because it harms the whole which we are part of, like a cancer cell.
Parable of the Video
Back in the old days people used to rent movies on VHS video tape. Unlike DVD's once this had been watched it needed to be rewound before it could be watched again.
A man hired a tape he found it was rewound he watched the film he rewound the tape he returned it to the shop
A man hired a tape he found it was not rewound he rewound it he watched it he did not rewind it he returned it to the shop
A man hired a tape he found it was not rewound he rewound it he watched it he rewound it again he returned it to the shop