The event-loop is the heart of ReactPHP, it pumps all the stream's data around, runs timers when they are due, and processes the ticks. But there is not just one, ReactPHP supports four out of the box.
The simplest feature in ReactPHP are ticks. Just as timers they are part of the event-loop. Ticks are often referred to as deferred callbacks. This makes them very useful to utilize the window between I/O events.