title: what is a `Scheduler` in RxJS
body: <p>I'v seen the term <a href="https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/schedulers/scheduler.md#rxschedulernow-schedule-schedulerelative-scheduleabsolute">Scheduler</a> very frequently in the documentation. </p>
  
  <p>But, what does this term mean? I even don't know how to use a so called <code>Scheduler</code>. The official documentation didn't tell me what a <code>Scheduler</code> exactly is. Is this just a common concept or a specific concept in RxJS?</p>