I get nervous at the thought of a test. Taking a test makes some students tense and anxious. A little bit of anxiety is useful. It helps you focus your attention and do well. Too much anxiety, however, is not helpful.
Some students score lower on a test because they are nervous.
Some react emotionally and feel stupid or like failures. If you have test anxiety, don't waste your time worrying about it. This is self-defeating. It won't help you do better on tests. Instead, keep your anxiety at a level that will help you, not hurt you, on the test.