According to Genesis, Noah coated the Ark with pitch. Genesis 6:14 says the vessel was to be covered with pitch inside and outside, making the hull watertight in the story.
Pitch is a sticky, waterproofing substance produced from organic materials such as wood or petroleum, depending on the historical process. The biblical Hebrew word is related to a term for covering or atonement, and translations commonly use “pitch” because the practical context is sealing the ark against water.
The instruction appears alongside the command to build the vessel from gopher wood. It is separate from the ark's dimensions and from its internal compartments. The narrative therefore gives both a proposed material and a protective coating, but it does not describe tools, construction stages, or a complete shipbuilding method.
Readers sometimes substitute tar for pitch. Those substances can overlap in everyday descriptions, but “pitch” is the standard concise answer for the wording of Genesis 6:14.