Genesis tells Noah to cover the Ark with pitch inside and outside. Genesis 6:14 gives this instruction after specifying gopher wood as the building material.
Pitch is a waterproofing substance used to seal wood and other materials. The biblical instruction therefore has a practical function within the story: it helps make the wooden structure watertight during the flood. The Hebrew wording is related to terms for covering or coating, and translations commonly use “pitch.”
The verse does not identify the exact chemical composition of the substance. Ancient waterproofing materials could include bitumen, plant-based substances, or other sealants depending on local resources and historical period. That uncertainty does not change the specific answer given by the Genesis text.