Perhaps the most natural transition, the pop is nothing more than a sequential landing into a unified takeoff. Simply put, you land with your feet in any order, and takeoff with both. This transition is a favorite because of its ease and efficiency in creating and maintaining power.
It should be noted that pop is also the name of a takeoff that is generally understood as any takeoff with both feet simultaneously, however transitionally, pops need to have a sequential landing first. In the early days of tricking, pop was also a stance that we now call frontside stance. There are still veterans in the community who use this term, but their numbers are dwindling as the mainstream increasingly adopts the term for use as a takeoff or transition, rather than a stance. It should be further noted that within mainstream terminology, when a backside kick is mentioned, it is usually an implied pop takeoff, and often a pop transition.