Lazy load is a method used to speed up the initial load time of a page and is especially effective on pages with lots of images below the fold.
The way it works is it only loads images that are on the screen when you first load the site, then continue to load images as you scroll just before they come in to view.
This is now a built-in feature of WordPress so you don’t need to do anything, but thought it was worth mentioning!
Lazy load doesn’t support background images though unless they use true parallax.