Sap was fluid present in various forms of vegetation, such as trees. It was a common ingredient in some prepared foods as well.
Oskoids contained sap that was said to help warm one's body. (TNG: "Ménage à Troi")
Jumja sticks were made from the sap of Bajor's jumja trees. (DS9: "In the Hands of the Prophets")
Sap from maple trees on Earth could be turned into maple syrup. (ENT: "Home"; VOY: "Workforce, Part II")