It means the union of the mind, the body, & spirit in communication with awareness. Whenever you attend yoga class, you will be asked to do different poses. These different poses are being done in order to help you relax and become more attuned with your surroundings.
The poses will help you learn more about how you will control your breathing. The time will come when you will not be mindful of your surroundings anymore. You can also improve your flexibility, your posture, and even your overall strength by practicing yoga. Yoga can calm you down and do so much more.