The wixData.save function isn't its own operation. It actually calls either wixData.insert or wixData.update, depending on what you're trying to save. Any item you want to save needs to have an _id property. Save checks if an item with that value already exists in your collection. If it does, we assume you're trying to update that item and save calls wixData.update with that item. If there is no item in the collection with that id, save calls wixData.insert and adds the item to your collection.
This is obviously useful if you're adding content to a collection and don't know if the item already exists. But don't forget that update overwrites your data with the new item. So make sure you include all the values for the item and not just the new ones. You can read about that in another tip, here.
