Debuff is the exact opposite of Buff. While Buff is a beneficial effect applied to you digimon or tamer, Debuff is a harmful effect that is applied to your digimon or tamer.
Currently, the debuffs are not very common in-game, as they are being added with the New Holy Beasts' Dungeons. Some Debuffs were in-game already, but in a low number.
The list of current debuffs in-game is down below: