Observer pattern in RxJava

Recently I'm trying to leverage RxJava in my projects and I find myself have difficulties in perceiving the spirit of its design.

I think RxJava is the extension of Observer Pattern. And if so, then there would be observers and observables in this pattern.

The mechanism work like this:

1. An observer subscribes to an observable.
2. When the state of the observable changes, it will notify its observer.

Here comes my problem:

As the RxJava official doc pointed out:

In ReactiveX an observer subscribes to an Observable.

but in the code of the Observable class,

public final void subscribe(Observer<? super T> observer) {
    ...
}

What is the meaning of the word "subscribe" here? Why don't they use "subscribe to" or "subscribed by"?