The answer box Google shows at the very top of some SERPs. 'Position zero.'
Featured snippets are short extracts (paragraph, list, or table) Google pulls from a page to directly answer a query. They appear above the regular #1 result, in a big box, and steal a lot of clicks from traditional rankings. Formatting content well (clear answer in the opening, lists where appropriate, tables for comparisons) can earn the snippet.
Featured snippets are some of the highest-CTR SERP real estate available. Structuring content to compete for them (clear question-and-answer formatting, lists, definition-style openers) is a high-leverage SEO practice.