All that we are is the result of what we have thought.
The mind is everything.
What we think we become.
Siddhārtha Gautama, Buddhism, Lumbini, India, Nepal
You become what you think about or in other words: what you focus on grows in your life and from what you take your focus away from diminishes. This is also the message of the law of attraction, recently very popular by the movie and book "The Secret ".
It means that the mind is directing our life in that way that we manifest the life around us by the thoughts and directions we take with our mind. Since everything we created around us is first created in the mind, it is the tool or the interface between our self and the material world.
Another quote of Buddha with a similar message here is "He is able who thinks he is able". This also shows the creative power of the mind and if we are able to consciously use it in that way – avoiding negative thoughts and utilizing empowering thoughts – we use it in a supportive manner.