Eustress is what we call positive stress in psychology. Imagine it as a "positive kick" that gets you moving and start working on something.
If you're an athlete or a musician, common experiences of eustress would be before and at the start of a game or show. You're feeling slightly nervous but you feel like you're up to the task and will be able to manage it. This sort of stress isn't crippling but rather beneficial. It is also true that not everyone would perceive stress in a positive way in such a scenario, for some such stress could prove harmful and wouldn't really help in their performance.
