It’s a reference to Noah’s Ark, where God supposedly flooded the whole world except two of each animal and Noah’s family, then put a rainbow in the sky as a symbol that He’d never do it again. Young Earth Creationists and their like take the story as fact, despite the fact that such an event would have put humanity well below the number of people needed for a stable gene pool, and that two of an animal likely wouldn’t repopulate an entire species. I digress.