Karma is the force of the Universe which balances life on earth. It is the force that drives people's actions: whatever you do today, will affect your future someday or the other. Everything eventually comes back to your sense of being, rectifying you. It's like everything is in a vibrating motion ...you pull the wave, it vibrates to and fro.

In other words, karma is directly related to the living beings on this planet. It is the reason behind everything and is the main reason behind new life & rebirth. 

As explained very vividly in Shreemad Bhagvad Gita: 'Never be scared to perform your actions, they are in your control. Leave the rest to God, he will take care of your Karma.' 

Everything you do could be traced back to Karma. 

Karma couldn't be purified fully by meditation but it could be improved. Yet, there are sayings that karma of past life could be purified through meditation because the mind contains all the psychological phenomenon. This practice is called 'Vajrasattva'. If one practices meditation and brings positive changes in life, his karma could be purified through meditation.

Meditation is the path of connecting with the inner self, the soul; and soul is the entity which passes from one life to the other  (resulting in rebirth) or attains salvation.  This is the reason why meditation is a useful way of purifying one's negative past.