With a simple example, let me make clear what one calls karma. Imagine, you work on anything from morning to night. Then you go to bed, sleep the whole night through, and get up in the morning again. If now you say to yourself, what I have worked yesterday does not concern me, I start afresh today, and then you are brainless, are you not? Nevertheless, the only possibility is that you take up in the morning again, what you have left in the evening, saying to yourself, this is my work and where I have stopped yesterday, I must resume today. What does that mean? That only means that my destiny of today is determined by my work of yesterday. Yesterday I have created my destiny of today. With it the whole concept of karma is given. Every being makes his future destiny.
Source: Rudolf Steiner – GA 54 – The Riddles of the World and Anthroposophy: Lecture XII: Reincarnation and Karma – Berlin, 15th February 1906