(covers information from several alternate timelines)
Prune juice was a sweet Human beverage made from the puréed dried fruit of a plum tree. It was known to help clear constipation of the Human digestive tract, and encourage digestive health in general.
Although generally unpopular among Humans for its bitter taste, the Klingon Worf was known to enjoy prune juice, which he proclaimed "a warrior's drink" when he was introduced to it by Guinan aboard the USS Enterprise-D in 2366. (TNG: "Yesterday's Enterprise")
Worf's Human adoptive parents, Sergey and Helena Rozhenko, were surprised when Guinan told them Worf enjoyed the drink, as he had never shown an interest in Human foods and beverages with them. (TNG: "Family")
Shortly after Doctor Bashir prescribed to Odo – who was, at the time, living in humanoid form – that he had to relax, Odo admitted, "And it helped.... That, and the prune juice." (DS9: "The Begotten")