It's a product or engineering document that defines how to build something that doesn't exist yet — going from nothing (0) to a first working version (1).
The name comes from Peter Thiel's concept of "0 to 1" (creating something new, vs. "1 to n" which is copying/scaling what already exists).
It's different from a spec or RFC in that it's less about technical detail and more about justifying why something should be built and what the minimum viable version looks like.
What does it cover?
Problem statement — what pain or opportunity exists
Proposed solution — what you're building and why this approach
Scope — what's in/out for the initial version
Key decisions — major tradeoffs and why you made them