Variance is the formal statistical measure of how far results spread around their average. In slots, it is the number behind the informal volatility label shown to players.
A game's maths model lists every possible outcome and its probability. The average of those outcomes gives the RTP; the variance measures how far typical outcomes sit from that average. Its square root, the standard deviation, is what studios and test houses use to work out how wide the range of likely session results will be.
Two practical points follow. First, variance shrinks relative to total stakes as you play more spins, which is why long-run results converge on the RTP while short sessions do not. Second, a game with high variance can show a big win or a big loss after the same number of spins, and neither tells you anything about the next spin.
People often use "variance" and "volatility" interchangeably, and in everyday use that is fine.