In star wars, the more "metachlorian" you have in your body.. the stronger you are with the force. Anakin Skywalker had more metachlorians than Yoda thus he was Stronger than Yoda.
The 'Force' is a mystical power that life creates. The 'Force' is said to comes from micro-cells called Metachlorians. The more Metachlorians you have, the stronger the 'Force' is within that 'Jedi'.