It is a sense of unease, possibly due to external factors like examinations or internal factors like lack of
esteem
It can also contribute to bad sleep.
It can also impair performance, but you will inevitably need and have a
bit of it.
Performance increases with physiological or mental arousal (stress), but only up to a point. When the pressure gets too high, performance plummets. Thus, you should aim to keep calm to an extent. Of course, zero stress means zero motivation to act, but too much stress means collapsing due to high pressure. Keep it under control. In fact, doing work often reduces stress as it literally reduces it.
A form of stress that actually is good. Only to a limited extent, because at the very simplest level, you need to recognize a threat, or a desire, and without such feelings, nothing will be done, so it's actually a good thing!
The worst type of stress is prolonged stress - where the brain gets no break from it for a long time.