Flow control is all about managing the rate of events so that a producer does not overwhelm its subscribers when raising a large number of events. A fast producer will push many events to its subscribers. Each of the subscribers will process these events as it receives them, one at a time. This sequential process can be quite inefficient, as each event is delivered over a wire.
In order to improve the efficiency, there are operators in Reactor that allow the producer to raise events in chunks. Each chunk of events is delivered to the subscriber, allowing them to work on many events simultaneously.