Express.js, or simply Express, is a back end web application framework for Node.js, released as free and open-source software under the MIT License. It is designed for building web applications and APIs. It has been called the de facto standard server framework for Node.js.
You should learn express or any js frameworks to understand nodejs, cluding: How nodejs works. Node package manage (npm) and middle ware. Practice new features of Javascript follow ES6/7 standart.