Slots volatility refers to a combination of a slot's payout frequency and the difference between its lowest and highest payouts. High and low volatile slots appeal to different kinds of players.
For example, a highly volatile slot doesn't payout often - but when it does, it's huge. You'll experience large runs of losing clicks followed by something big and beautiful. A low volatility slot, on the other hand, has more frequent wins per clicks, but the wins are modest at best. Your chances of hitting it big on a low volatility slot are small compared to a highly volatile slot.