From the Node.js official website (https://nodejs.org):
Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine. Node.js uses an event-driven, non-blocking I/O model that makes it lightweight and efficient. Node.js' package ecosystem, npm, is the largest ecosystem of open source libraries in the world.
Node.js is widely used as a backend for web applications because it is easy to create an API and its performance is better than technologies such as Java, PHP, or Ruby. Usually, the most popular way to use Node.js is by using a framework called Express.
From Express official website (https://expressjs.com):
Express is a minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications.