Volatility describes the pattern of a game's wins, how often they happen, and how big they tend to be.
With low volatility, you'll win more frequently, but the amounts are usually small, so the game feels steady and consistent.
With high volatility, wins come less often, but when they do land, they can be much bigger. So the game feels more up-and-down, with longer stretches of nothing followed by a bigger payout.
Neither is "better," it's really about the style of play someone prefers: steady and frequent versus bigger but rarer.