In RxJava an object that implements the Observer interface subscribes to an object of the Observable class. Then that subscriber reacts to whatever item or sequence of items the Observable object emits. This pattern facilitates concurrent operations because it does not need to block while waiting for the Observable to emit objects, but instead it creates a sentry in the form of a subscriber that stands ready to react appropriately at whatever future time the Observable does so.
For information about the Observable class, see the Observable documentation page at ReactiveX.io.