https://www.cnet.com/news/facebooks-new-why-am-i-seeing-this-post-wants-to-make-sense-of-your-feed/

Though it was announced a day prior to April 1, the new feature is not a April Fool's Joke, a Facebook spokesperson said to CNET. It'll begin rolling out this week and be available to all Facebook users by mid May, the spokesperson said. 

The function will show you what data was used to prioritised the post on your feed. How often you like a person's posts and how often you engage with different mediums like video, photo or text were two given examples of information used to shape a post's prominence.