According to Genesis, pitch sealed Noah’s Ark against the floodwater. Genesis 6:14 tells Noah to cover the Ark with pitch inside and outside. The coating is presented as waterproofing for the wooden vessel before the flood begins.
Pitch is a generic term for a thick, sticky, water-resistant substance historically obtained from petroleum, coal, wood, or other organic sources. The biblical Hebrew word is related to terms for covering or coating, and translations commonly render it as pitch, bitumen, or a similar waterproofing material.
The instruction is distinct from the Ark’s construction material. Genesis names gopher wood for the structure and pitch for the protective coating. Because translations vary, “bitumen” may appear in some versions, but “pitch” is the traditional English answer to the wording about what sealed the Ark.