These are very specific cooking words and certainly not things that we say often. 'Poach' is a way of cooking food, usually eggs or fish, which means cooking them whole / in large pieces in water at a low temperature. https://en.wikipedia.org/wiki/Poaching_(cooking) However, 'simmer' means to cook anything wet using a medium heat. It refers to the whole pot, not just the sold things inside the liquid. You can do it to water, a sauce, a soup, stew, or anything else liquid. It is common in recipes to "bring [something] to a boil" and then "let it simmer for 20 minutes".

Poaching refers to a method of cooking where something is cooked in a little water. But simmer refers to allowing anything to boil gently. Example: to poach an egg, allow it to simmer gently in the water. But Tea, soup or stew can simmer.