When something's stretched tight, it's tense. A rope can be tense. So can a muscle. When a person's tense, they're anxious.
When a situation is tense, you might say you could "cut the tension with a knife." When a muscle tenses up, it contracts suddenly–-a person can be said to tense up also. The opposite of tense is "relaxed." If you tense up during exams, it's probably best to learn some relaxation techniques. The brain works best when it's relaxed.