What does the populate function in mongoose do?
In Article document, you are storing userId of the users.
Now while querying Article document, with articles you want to get users for each then you can use populate.
Now when you access articles[index].user, then it will return object instead of ObjectId value (_id property of user).
Second argument defines that which field should be retrieved in User object. Here it is displayName. So it will retrieve _id and displayName of User. If you want to exclude _id you can specify "-_id displayName"