It is when you are so nervous about preparing or taking a test that you have difficulty planning, thinking, concentrating, recalling what you have studied, and relaxing. It is common, understandable, and curable. We need a little anxiety to get us going. If we had no feelings of anxiety, we wouldn't care and we wouldn't get anything done. However, it is not good when we have so much anxiety that we set ourselves up for failure.
Fear can be our enemy.
Take care of yourself. Organize your time and stick closely to it. When you are to study, study. Allow time for recreation and study breaks. When you schedule yourself to have fun and relax, enjoy.