Position zero is the seat at the head of the table, above the so called number one result. It is usually a featured snippet, and it is some of the most valuable space in all of search.
The short version
Position zero is the informal name for the result that appears above the first standard organic listing, most often a featured snippet. It answers the query directly at the very top of the page.
Why it is worth chasing
It sits above everything else organic
It often feeds voice search answers
It builds authority by being the chosen answer
How to target it
Answer the question concisely and early
Match content format to the query, list, table, or paragraph
Use clear question based headings
FAQ
Is position zero always better than position one?
For visibility, often yes, since it sits higher. But it can also mean users get the answer without clicking, so weigh it against your goals.