This usually refers to getting a hit (particularly a home run) when the player's team is trailing late in a game and needs to score to tie or take the lead, especially if there are already two outs in the inning and/or the batter already has two strikes.
Since Cramer published his results, others have similarly searched for evidence that clutch hitting is a skill, but these studies have only tended to confirm Cramer's findings: the notion that certain players are consistently able to raise their level of play in high-stakes situations is an illusion.
In an interview with Sports Illustrated, New York Yankees shortstop Derek Jeter criticized analysts who deny that clutch hitting is a skill, saying: "You can take those stat guys and throw them out the window."
Though widely considered a clutch hitter (and so nicknamed "Mr. November"), Jeter's career batting average, on-base percentage, and slugging percentage (BA/OBP/SLG) are .317/.388/.462, while the same statistics over Jeter's career postseason play (when the stakes are higher) are .309/.377/.469, so either slightly worse or not meaningfully better.
This is not to say that it is impossible for a player's psychological handling of high-pressure situations to impact (positively or negatively) their performance, i.e. confidence in one's clutch hitting ability begetting clutch hits or a lack of confidence begetting "choking."