Volatility (sometimes called variance) describes how a slot's payouts are distributed over time — not how much it pays overall (that's RTP), but the pattern of when and how large those payouts tend to be.
Low volatility
Frequent, smaller wins. Your balance moves in smaller steps, up and down, with less dramatic swings. Better suited to a smaller bankroll or a session where you want to play for longer without large ups and downs.
High volatility
Infrequent but potentially much larger wins, with longer stretches of no win in between. Requires a larger bankroll and tolerance for extended losing streaks between hits, in exchange for the chance at bigger individual payouts.
Medium volatility
A middle ground — moderate win frequency and moderate payout size, without the extremes of either end.
Volatility and RTP work together, not separately
Two slots can share the same RTP (say, 96%) with completely different play experiences — one paying out often in small amounts, the other rarely but bigger — because RTP describes the long-run total, while volatility describes how that total is distributed along the way. Neither figure alone tells the full story.
Matching volatility to your bankroll
A small bankroll on a high-volatility slot risks running out of funds before hitting one of the bigger, less frequent payouts. Matching volatility to your actual bankroll and session length is a more useful practical decision than optimizing for RTP alone.