Incomplete ideas, thoughts and jottings
Before you sit down to write a first draft, you need a sense of what you're going to write about. But what if you've got no idea? That's where the 0th draft comes in. It's exploratory writing that doesn't matter, free writing. You know you are going to throw it away. It's a draft before you actually start the main project, so it counts a draft 0. It's following every blind alley to see where they are going to end up. It's writing to find out what you want to write about.