Volatility describes how spread out a slot's results are. Low-volatility games return frequent small wins; high-volatility games go long stretches with little and concentrate their return in rare large wins.
Crucially, volatility is about the spread of outcomes, not the average. Two games with the same RTP cost the same in expectation per pound staked, but a high-volatility one makes that cost far less predictable over a session. It is usually shown as a label (low, medium, high) or a scale of bars, because studios do not publish a single standard number.
Volatility tends to go with a lower hit frequency and a higher max win, since a game that can pay a very large multiple has to fund it by paying less often elsewhere.
High volatility does not mean a game is "due" after a dry spell. Each spin is independent. Our volatility simulator shows how the same RTP can produce very different sessions.