It means that you attract those things that are in alignment with your most dominant thoughts and beliefs, those things which you think about with desire and conviction.

As a practical example, you tend to hang out with (are attracted to) people who are "like-minded," who have similar interests. Likewise, they are attracted to you.

The phrase describes an aspect of the creation of your own reality, the process whereby you imagine something being in your life, and the universe responds in turn, making circumstances and opportunities available to you to help you achieve that thing. By focusing on what you want, you begin paying attention to those things in your environment that will help you get what you want, and matching those things that you encounter with your focused desire.