Infinite Scroll is a module of the Jetpack plugin.
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.
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.
