If you’re not entirely sure what the difference between simmering and boiling water is, this analogy will give you a much better idea.
There’s a similar Chinese analogy, associated with gung fu tea preparation. “Shrimp eyes” is the pre-simmering temparature, with bubbles the size of…shrimp eyes. “Crab eyes” or “fish eyes” is simmering. “Dragon eyes” is a full boil.