Volatility (or variance) describes how risk is distributed in a game. Low-volatility slots pay small wins often; high-volatility slots pay rarely but much bigger.
Which should you pick?
If you want long sessions and steady play, choose low or medium volatility. If you chase big multipliers and can handle dry spells, high volatility suits you. RTP tells you the long-run return; volatility tells you the ride to get there.
Match volatility to your bankroll — high-variance games need a larger buffer to survive losing streaks.