How does Node.js work? For examle, is it more scalable than standard threaded web servers due to the lack of threads locking and context switching?

I don't understand several things about Node.js. Every information source says that Node.js is more scalable than standard threaded web servers due to the lack of threads locking and context switching, but I wonder, if Node.js doesn't use threads, how does it handle concurrent requests in parallel?

What does the event I/O model means?