Genesis tells Noah to coat the Ark with pitch inside and outside. In Genesis 6:14, the waterproofing instruction follows the command to build the vessel from gopher wood.
Pitch is a dark, waterproofing substance produced from tarry or resinous materials. In an ancient boat-building context, a coating of this kind would help seal joints and reduce water penetration. The biblical Hebrew word is related to the idea of covering or atonement, creating an additional word connection often discussed by interpreters.
The instruction does not identify a modern commercial product or specify one exact chemical recipe. Translations may use terms such as pitch, bitumen, or tar. These words describe the waterproofing function, while “pitch” is the traditional English answer to the Genesis question.