Karma (Car-ma) law means Recurrence.
It means everything you do in your life, every action we do, every word we say and so on will reflect to us.
karma is cause and effect theory.
If you make someone happy in your life then you will be happy somewhere else in the world and vise versa if you make someone sad it will reflect to you in a different way.
YOU WILL GET THE SAME FROM EVERY HAND
Image source:
http://www.radhanathswamionline.com/insights/governed-laws-karma-choice-life-radhanath-swami-answers/