This quote means that your emotions are controlled by your thoughts, and your thoughts are controlled by your emotions. Basically, it is saying that your emotions are driven by what you are thinking, and your thoughts are driven by the emotions you are feeling. So your emotions and thoughts are linked, and one has a direct effect on the other. Ultimately, this means that you have some level of control over the way you feel and think because you can choose to focus on different thoughts and emotions.