The 8-ball is a concept to create new patterns of steps based on figure eight. I heard about this idea the first time from b-boy Alieness who taught an 8-ball six step at Circle Prince Croatia. I can not remember the exact year, to be honest.
While there are multiple different 8-ball steps (like the aforementioned 8-ball six step), the concept follows only one simple rule:
You take a series of steps/moves and combine them in a way that they flow in both directions without interruption.
In the original idea, the creators tried to emulate the figure eight as a path on the floor. Today most people consider a step to be an 8-ball when you follow the rule above.
And because it is easier to understand when you see it, I embed a video from Poe One, from Style Elements crew, below. He demonstrates multiple steps that were created with the 8-ball idea in mind.