The event loop is a fundamental concept in Node.js, and is responsible for executing the JavaScript code in a non-blocking, asynchronous manner.

In Node.js, all I/O operations are performed asynchronously, which means that they don’t block the execution of the main thread. Instead, these I/O operations are registered with the event loop, and the event loop will call the callback function when the I/O operation is complete

This allows the program to continue executing other code while waiting for the I/O operation to complete.

The event loop in Node.js consists of several phases, including the timers phase, the pending callbacks phase, the idle, prepare phase, and the poll phase. In each phase, the event loop will process a different set of tasks. For example, in the timers phase, it will process timers that have expired, and in the poll phase, it will process I/O events.

It’s important to note that the event loop is a single-threaded loop, which means that only one task can be processed at a time. However, due to the non-blocking nature of Node.js, the event loop can efficiently handle multiple I/O operations in parallel, which gives the illusion of multithreading.

In summary, the event loop in Node.js is the mechanism that enables the program to execute JavaScript code in a non-blocking, asynchronous manner, by processing I/O operations as events and calling the corresponding callback functions when the I/O operations are complete.