Karma: the most simple explanation is "what you sow you will reap'  meaning that all acts of goodness you do will be rewarded in kind sometime. If you are a really lousy, selfish bad person then eventually all your acts of 'badness'  will catch up with you!! sometime, somewhere eventually!!

The idea of Karma originated in India.. in a nutshell people who have good karma are empathetic and sympathetic but it does not mean they are weak and that others take advantage of them!!