What is position 0 in Google Search?

Position zero is the information Google offers at the top search results. (Ads don't count as they always come first...)

Position zero is also called the featured snippet, and is precisely that: a featured snippet of content that directly answers a search query based on the "most relevant immediate answer". 

Position zero is placed above the top-ranking search result and displayed differently.

Here are a couple of examples:

Or also: