An item is an aspect of the Dungeon Dice Monsters game. They are placed in the dungeon on the die they are contained in is unfolded.
In the manga, items exist within rare black dice, as apposed to standard black dice. A rare black die contains both a monster and an item crest. The item crest is fixed to the inside of one square of the die. In the anime, a rare black die contains an item, but no monster.