export default in javascript

I'm working through a tutorial on how to create a restful api using express and mongoose.

I'm understanding everything except for this bit right here

import mongoose from 'mongoose';
import config from './config';

export default callback => {
    // connect to the database
    let db = mongoose.connect(config.mongoUrl);

    // callback db
    callback(db);
}

I understand what its purpose is and how to call it. What's confusing me is the callback(db) part.

Is that a built in function or is it somehow calling itself?

via Tucker Beauchamp