Variance
Variance is the natural swing between your short-term results and your long-term expectation, caused by the luck of which cards come out.
In poker, variance describes how much your actual results bounce around your true skill level over any short stretch of hands. Even if you make the right decision every single time, the cards still fall randomly, so you can run far above or below what you "should" win for a while. The better your decisions, the more those swings even out and trend toward your real expectation the more you play.
It shows up everywhere at the table. You can get your chips in as a big favorite and still lose, or chase a long-shot draw and hit it; one session can soar while the next sinks, all without your play changing at all. A run of bad luck against the odds is what players call a bad beat, and it is pure variance in action, not a sign you did something wrong.
For beginners, the key takeaway is to judge yourself by your decisions, not by any single result. A good fold or a smart value bet is correct even when it loses that hand, because you are betting on what happens over thousands of spots, not one. Keeping that long view also helps you avoid tilt, the frustrated, reckless play that bad swings can trigger.
Example
You move all in pre-flop with pocket aces and get called by K-Q suited; your aces are about an 82 percent favorite, yet the board runs out with a queen and a king to make two pair and you lose. Your decision was great, the cards just swung against you, that is variance.