The stream is one of the most significant module among the core modules. To define the stream in simple words, it is the flow of data received as small data chunks from a particular source. At the receiver's end, it might neither have all the stream data nor it will have to fit it memory all at once. This allows us to handle a large amount of data with limited resources. We can visualize the stream by an analogy given by Dominic Denicola. According to him:

Consider if we need to read a large file of data in the environment where multiple read/write operations takes place. In such cases, streams provides a powerful abstraction to handle the low-level I/O system calls while still providing performance benefits.

Although, streams are not extensively used in context of our book, it is one of the backbone feature of Node.js core used by its internal modules itself and has always been a big part of Node.js ecosystem. To learn more about streams follow the link: https://community.risingstack.com/the-definitive-guide-to-object-streams-in-node-js/.