As the website says: "Lazy loading boosts page speed by deferring the loading of images until they're in (or near) the viewport. This library makes it completely painless - maximizing speed by keeping options to a minimum."
Implemented this on my site earlier this week after coming across it, there are quite some lazy loading libraries but this one is very well executed from a "UX" standpoint as well. The images are lazy loaded slightly under the viewport, making it feel as if all images were there already once you get there, while greatly reducing page load times (if you have a lot of images that is).
