Washing your mouth out with soap is when someone puts soap in their mouth and then spits it out or rinses it out with water. It's usually done as a punishment when someone uses bad or disrespectful language.
So, imagine you said a bad word or were being mean to someone, and your caregiver heard you. They might tell you to go over to the sink, and they’ll grab a bar of soap. They'll tell you to open your mouth and stick out your tongue, and then they'll rub the soap on your tongue and teeth.
This might taste really bad, and you might not like it. But the idea is that you'll learn your lesson and not use bad words or be disrespectful again.
It's important to note that washing out your mouth with soap can actually be harmful and make you sick if you ingest too much soap. So it's important that it's not done too often or with too much soap.