Parallelization, also called parallelism or parallel computing, is a broad term that can be used for any concurrent activity (including what we covered). But for the purposes of RxJava, let's define it as processing multiple emissions at a time for a given Observable. If we have 1000 emissions to process in a given Observable chain, we might be able to get work done faster if we process eight emissions at a time instead of one. If you recall, the Observable contract dictates that emissions must be pushed serially down an Observable chain and never race each other due to concurrency. As a matter of fact, pushing eight emissions down an Observable chain at a time would be downright catastrophic and wreak havoc.
This seems to put us at odds with what we want to accomplish, but thankfully, RxJava gives you enough operators and tools to be clever. While you cannot push items concurrently on the same Observable, you are allowed to have multiple Observables running at once, each...