This quote is saying that if you take control of the things you can change in your life, you can make incredible changes. This means that instead of focusing on the things you cannot control, you should focus your energy on things that you can. For example, you may feel that you cannot control the situation at work, but you do have the power to take the steps necessary to improve your skills and further your career. Taking control of what you can change will help to bring about positive changes in your life.