Noah used pitch to coat the Ark inside and outside. Genesis 6:14 instructs Noah to cover the Ark with pitch, a waterproofing material, on the inside and outside.
Pitch is a thick, sticky substance traditionally produced from tar, resin, or related materials. Its water-resistant properties made it useful for sealing wooden boats and protecting wood from moisture. The instruction fits the Ark's description as a wooden vessel intended to survive prolonged flooding.
Different translations use words such as tar, bitumen, or pitch because the underlying ancient Hebrew term can be rendered in more than one way. The central idea is a waterproof coating, not a particular modern commercial product.
The verse also specifies gopher wood and gives the Ark its dimensions. Pitch is therefore one of several concrete construction details in the Genesis account, alongside rooms, a door, and a window.