An event loop is basically a single thread that executes functions from a FIFO queue. A function can put itself on a queue by "yielding", which means, allowing other functions to execute.

If your function blocks, for example, by performing a wait on something without yielding, then nothing else gets computed because of the wait, but the event queue is occupied since the function has not yielded. This breaks the cooperative part of the cooperative multithreading mechanism of Node.