Short definition
IndexNow is an open protocol from Microsoft and Yandex that lets a website ping search engines the instant a URL is created, updated, or deleted.
In depth
IndexNow shortens the indexing lag from days to minutes. The publisher generates a key file at the site root and POSTs URLs to the IndexNow endpoint when content changes. Bing, Yandex, Naver, Seznam, and others honor it; Google does not officially participate but the broader ecosystem benefits. For a content-heavy site, IndexNow combined with sitemaps and a clean robots.txt is the modern indexation stack.
Example
An agency wires its CMS to IndexNow. New blog posts appear in Bing search within four minutes versus the prior 36-hour average.