Okay that's simple, now let's look a little at the process in the cylinder. Basically you can count on two engine processes: a two stroke or a four stroke engine. Which means it takes or turns to return to the beginning position of powering the motorcycle. As you can imagine having only two strokes makes it possible to power the motorcycle much more regularly. However a two stoke engine is more reactive and more nervous to drive. A four stroke is a little more laid back.
Back to how it works. As gas and air get sucked into the cylinder the piston comes around and squeezes this gas mixture together. At the most effective time the sparkplug ignites the mixture, which sends the piston back down. The force pushes the shaft around. Now multiply this process by a few cylinders and raise the rate at which it happens and you got yourself a powering engine driving you down the road.