By Alvin Alexander. Last updated: June 6, 2016
According to the URL shown, here's the difference between save and insert in MongoDB:
For save, if you provide _id, it will update. If you don't, it will insert.
There is additional documentation at the bottom of that thread:
From this, we can see that save is a wrapper for update and insert. Functionally, save and insert are very similar, especially if no _id value is passed. However, if an _id key is passed, save() will update the document, while insert() will throw a duplicate key error.