What is happening

An asynchronous computation cannot provide a result immediately when it is started, unlike a synchronous computation which does compute a result immediately by either returning a value or by throwing. An asynchronous computation may need to wait for something external to the program (reading a file, querying a database, fetching a web page) which takes time. Instead of blocking all computation until the result is available, the asynchronous computation immediately returns a Future which will eventually "complete" with the result.