实例方法如何在Mongoose中工作？

I'm working on a course in building a REST API with Mongoose. We have a question and an answer document and they're showing how to update the answers to the questions collection using an Instance Method on the Answers Schema. I'm not able to follow along at this point because, i can't tell what the callbacks being passed in as arguments are for and what they're exactly doing to the schema.I'm working on a course in building a REST API