When something is unbalanced, it's uneven or unstable. If your weight on a surfboard is unbalanced, you risk falling off.
Things can be physically unbalanced, like a seesaw with only one kid on it, or more figuratively unbalanced. For example, your mom might think you have an unbalanced diet if you rarely eat vegetables, and you'd describe your high school education as unbalanced if you didn't take any math classes. Another way to be unbalanced is to be mentally ill or emotionally disturbed.