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All your application's JavaScript code runs on a single main thread called the Event Loop.
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Non-Blocking: Because it is single-threaded, Node.js uses asynchronous, non-blocking I/O to handle thousands of concurrent connections without the overhead of creating a new thread for every request

The Multi-Threaded Aspect: While your code runs on one thread, the Node.js runtime environment uses multiple threads 'under the hood' to handle heavy tasks:

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Libuv Thread Pool: Node.js uses a C++ library called libuv, which maintains a default pool of 4 background threads (configurable via UV_THREADPOOL_SIZE).
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Internal Tasks: These background threads handle intensive operations that would otherwise block the main thread, such as File System (I/O) operations, Cryptographic functions (e.g., hashing), Compression (Zlib), DNS lookups.
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V8 Engine Threads: The V8 engine itself uses separate threads for tasks like garbage collection.