IndexNow is a protocol that lets a website tell search engines straight away that a URL has been added, updated or deleted. Instead of waiting for a crawler to visit, you send a notification yourself. Participating search engines share notifications with each other: notify one and you notify them all. According to the IndexNow FAQ, they include Bing, Yandex, Naver, Seznam.cz, Yep and Amazon. Google is not on that list.
How does it work?
You place a key file in the root of your site, so the search engine can verify that the notification comes from you. Then you send a request with the URL and your key. According to the documentation, a single request can submit up to 10,000 URLs.
Example
https://api.indexnow.org/indexnow?url=https://www.example.com/offer&key=your-keyIn practice you rarely do this by hand. Many systems and plugins send the notification automatically when you publish or update a page.
Common mistake
Thinking a notification guarantees inclusion. IndexNow states that a 200 response only means the search engine accepted the submission. Each search engine decides for itself whether and when it crawls and indexes the page. For Google, an up-to-date XML sitemap remains the way to submit new URLs in bulk.
Bing Webmaster Tools shows how Bing processes your site.