Handling tornadoes and asynchronous requests

My question has two parts:

• What exactly does an “asynchronous server” usually called Tornado mean? Can someone provide a specific example to illustrate the concept / definition?

• In the case of Tornado, what exactly does “non-blocking” mean? Is this due to the asynchronous nature of the above? In addition, I read it somewhere, it always uses one thread to process all requests, does this mean that requests are processed sequentially one after another or in parallel? If the latter is the case, how does a tornado do it?