Clustering in Node.js is a technique of replicating the same Node.js process multiple times to utilize all the available cores of a CPU, thereby making the application more scalable and efficient. Essentially, the Node.js Cluster module enables the creation of child processes to handle incoming requests while the main thread remains free to accept new requests.

This results in faster response times, as the work is divided among multiple processes and can be executed in parallel. Clustering in Node.js is particularly useful for applications that receive a high volume of traffic.

The Cluster module in Node.js provides an easy-to-use API to create a cluster of Node.js processes. The master process creates child worker processes, each of which is a replica of the master process, and can handle requests independently. The master process also listens to messages sent by worker processes to monitor their activities.

By default, Node.js uses the number of cores available in the system to create worker processes. However, developers can customize the number of worker processes created for the cluster based on their application needs.

Clustering in Node.js can help in scaling applications that may have started as a single-process application. With clustering, you make sure that you take advantage of all the cores available in your system, meaning that your application can handle more traffic easily. As a result, user requests are processed faster, and the overall efficiency of the application is increased.