Asynchronous applications are programs that can process events, requests, and tasks with time slicing so that one request can be active and run while another one is also but not running:

Modern web servers, As shown in the preceding diagram, an asynchronous application can accept and delegate multiple requests threads/processes to keep accepting requests for a certain level. While doing this, the application will usually have a queue to hold received yet unprocessed requests up until threads to process them become available.