It's a Shinto creation myth. They created the Japanese islands but one day after Izanami gave birth to a fire god she died
Izanagi went to the underworld to get her back because she couldn't leave once she'd eaten food from the underworld. Izanami was ashamed of how she looked in the underworld, she was presumably rotting and grotesque. Izanagi was like 'nah, I still love you. Just let me light a fire so I can see you-' so he did and ran away from her once he saw her. She was like "
I'm going to kill 1,000 of your people everyday because of what you did" and he was like "Fine. Well I'll just create 1,500 more people every day."
That's the very basic and 'Ali's tired story telling' version of the myth.
The World Serpent, aka Jormungandr, is one of Loki's children and he grew so big that he could fit around the world. When he unwraps his tail it means the start of Ragnarok.
(If you watched Jacksepticeye play God of War you might've heard of him!)