Let's look at a very simple creation operator, of(). This takes a variable number of arguments, which will be emitted as values, like so:

let stream$ = Rx.Observable.of(1, 2, 3, 4);
stream$.subscribe( data => console.log(data)) // 1, 2, 3 ,4

The values are fired off immediately. This is very useful when you just want to test things out.