Variance is the spread of outcomes a player should expect around an average result over a fixed number of hands. It is a property of the game, the deck and the rules, and it exists whether or not any individual player is at the table. The Supreme Court of India described rummy as mainly and preponderantly a game of skill because the player's choices shift the average over a long run; variance is what makes the short run noisy around that average.
A useful way to read variance is as the width of a window. Imagine the long-run win rate of a deliberate player is a fixed point on a line. The results of any particular session are scattered around that point, with most sessions landing close to it and a smaller number landing far away. The scatter is variance. Over a single session, the scatter is wide enough that a player cannot tell where the centre is. Over many sessions, the scatter narrows and the centre becomes visible.
The honest player learns to ask, before reacting to a result, how many hands it represents. A single bad session can come from the natural width of variance. So can a single great one. The arithmetic that follows turns that intuition into a number, and the number is the difference between reading a streak and reading skill.
What variance does not say
Variance does not tell a player that the game is rigged, that the deck is stacked or that an opponent is cheating. Those are different questions, with different evidence, and they sit on a different page. Variance is what the game itself produces, hand by hand, regardless of who is dealing.