In the tale Goldilocks and the Three Bears, Goldilocks tries all of the bears’ porridge, chairs and beds, finding that only the little bear’s things were just right. Everything else was a little off for her…too hot or too cold, too hard or too soft and so on.
Similarly, for everything to work as it should in the body, things need to be just right. Blood pressure shouldn’t be too high or too low; organs can’t be too big or too small, etc.