Express is the most popular web server for Node.js and is described on its own website as a "fast, unopinionated, minimalist web framework for Node.js". It is also said to be open and flexible, allowing you to write only the code that you need, and nothing more. It is designed for building web applications and APIs.
Express can also be used with a number of different templating engines. The role of a templating engine is to provide static template files in an application, which contain variables that are replaced with actual values at run time and are then served up as HTML to the browser. Templates are special types of files that use their own syntax and language. Some examples of templating engines that work with Express include Pug, Mustache and EJS.