This jetpack feature loads more posts when you get to the bottom of a page of post.
To se it on action just go to infinite scroll and scroll down the page. More posts will load until you get to the very first post.
To use the Feature:
Activate the Jetpack Plugin
Activate the Infinite Scroll module in the Dashboard->Jetpack section
Instead of having to click a link to get to the next set of posts, infinite scrolling pulls the next posts automatically into view when the reader approaches the bottom of the page.
The Infinite Scroll module will not work if you also active the Jetpack CSS module.
NOTE: Most themes nowadays have Infinite Scroll support built in already! To check to see if your theme supports Infinite Scroll, simply activate the Infinite Scroll module under Jetpack -> Settings and check that “Scroll Infinitely” is checked under Settings -> Reading.