In basketball, there are 5 major stat categories: points, assists, rebounds, steals, and blocks. A triple double is when a player achieves double digits in 3 of these categories during a game. The triple refers to the 3 categories and the double refers to double digits (10). Players who achieve triple doubles are usually very versatile, as they can have a huge impact on many aspects of the game.
Triple doubles are quite rare in basketball, as it is harder than you would think for a player to reach 3 categories of 10+. A typical statline for a triple double could look something like this: 18 points, 11 assists, and 12 rebounds. Although most triple doubles include points, rebounds, and assists, it is possible to achieve a triple double using blocks or steals. So long as at least three of the categories are above 10, then it counts as a triple double.