Dice jail is a place where players can put dice that roll badly (causing negative consequences for the player characters, e.g. a critical fail). Dice that are put in dice jail are not used for subsequent rolls, perhaps for the rest of the session, and is therefore usually only a part of culture for players who have large dice collections. Dice jail is connected to the concept of dice shaming.
Dice jail might be metaphorical (an intention to roll different dice instead of the offenders) or physical (a sort of game aid, either handmade or bought as a novelty).
Dice jail is generally a tongue-in-cheek reference to dice superstitions that anthropomorphise dice ("if dice roll badly, it is because the dice are malevolent, so they should be punished"), but because of potential imbalance and unfairness from the dice manufacturing process, the most likely roll on a die (about which no other rolling probabilities or fairness are known) is what it has rolled before. As such, in theory, dice that roll badly are more likely to roll badly in future, and isolating those dice may be a way to try to cultivate a collection of dice that roll well without intentionally using weighted dice.