Zaqqum is a type of tree that grows in hell. It's fruit (A-dhari) resembles the heads of devils and its taste is very bitter. The sinners of hell will be in constant hunger and feel compelled to eat from the fruit which tastes horrible and will boil their bellies like boiling water.

The Holy Quran, Surah 44 (The Smoke)

[44.43] Surely the tree of the Zaqqum,
[44.44] Is the food of the sinful
[44.45] Like dregs of oil; it shall boil in (their) bellies,
[44.46] Like the boiling of hot water.