Usually when you're making a soup, you want to bring it to a boil and then let it simmer, cooking it just below the boiling point to get all the flavors to release.
The word simmer is usually used to describe the temperature just below boiling or the process of cooking something at that temperature. But it can also be used to describe a feeling that's been simmering or slowly developing over time. It's also a word used to describe suppressed anger — that feeling just before you boil over. Just in case you do boil over, you might need to take a moment to simmer down, a casual expression that means, "Chill out, my friend."