According to Genesis, pitch seals Noah's Ark against water.
Genesis 6:14 tells Noah to cover the Ark inside and outside with pitch. The instruction appears alongside the command to build the vessel from gopher wood and divide it into compartments. Pitch is a waterproofing substance, so the detail gives the Ark’s construction a practical function within the story.
The original language uses a word related to covering or coating, and translations commonly render it as pitch, bitumen, or resin. This variation does not change the basic narrative idea: the vessel is sealed against water on both sides. The text does not identify a particular modern petroleum product or give a recipe for making it.
Pitch is sometimes confused with the wood used to build the Ark. They are different materials: gopher wood is the construction material, while pitch is the waterproof coating. The same Genesis passage also mentions internal rooms and multiple decks.