According to Genesis, Noah was told to apply pitch to the Ark to make it watertight. Genesis 6:14 says the vessel was to be covered with pitch inside and outside after it was built from gopher wood.
Pitch is a broad term for a sticky, water-resistant substance derived historically from materials such as bitumen, wood, or resin. The Hebrew wording is related to a root meaning “to cover” or “to smear,” and the substance’s precise chemical identity is not established by the passage.
Popular retellings sometimes use “tar” as if it were the exact original word. Tar is a plausible general description of a waterproofing material, but “pitch” is the conventional English rendering in many Bible translations. The instruction is practical and distinct from the Ark’s structural material: gopher wood formed the vessel, while pitch sealed it.