Let's see a real example with the map operator. This operator takes a source observable as input and returns an observable as output. It applies a function to each item of the source observable, and emits the result of this function on the output observable. Hopefully, its marble diagram, shown in the following figure, should make the description much more clear:

The marble diagram shows an input timeline with three integer items: 1, 12, and 7. The transformation is described with syntax similar to that of the Python lambda. Note that the official ReactiveX documentation uses the JavaScript arrow function notation, because it uses marble diagrams from the JavaScript implementation. In this example, the transformation is a multiplication by 3 of the source item. The output timeline contains also three items, corresponding to the input items values multiplied by 3.

The prototype of this operator is the following one:

Observable.map(self, selector)

The selector parameter is the function that will be executed on all items of the input observable.