Horklump Juice is a magical liquid ingredient used in Potions. Its specific colour and consistency are not described in the novels. The juice is extracted from Horklumps, which are magical creatures native to Scandinavia that resemble pink, fleshy mushrooms covered in sparse, wiry black bristles (Fantastic Beasts and Where to Find Them).
Horklump Juice is primarily known as a Potion ingredient. Its most notable canonical use is as a component in the Draught of Living Death, an extremely powerful sleeping potion taught to advanced Hogwarts students. According to Fantastic Beasts and Where to Find Them, the Horklump creature possesses healing properties, which strongly implies the juice is used in various healing potions. This is a common application in supplementary materials, where it is often featured as an ingredient in restorative draughts like the Wiggenweld Potion (video game).
Horklump Juice is mentioned on two key occasions, serving to add detail to the world of Potions and to facilitate specific plot points.