Simmering is like when you're making soup on the stove. You put all the ingredients in the pot and turn the stove on to make it hot. But then you don't turn the stove all the way up to the highest setting, because that would make the soup boil really hard and splash everywhere. So, instead, you turn the stove down to a medium or low setting, and that's called simmering.
When you simmer something, it means you're heating it up slowly and gently. The bubbles in the pot will be small and not as intense as when you're boiling something. This is good for soup because it allows all the flavors to mix together and cook slowly. It also helps make the meat or vegetables in the soup tender and juicy, instead of tough or overcooked.
So, whenever you see a recipe that says to "simmer for 30 minutes" or something like that, it means to cook it gently over a low heat. This will make your food taste better and be healthier for you. Yum!