When a page is coded in such a way that the page components load only when the user needs them, rather than all at once upon the first page load. For example, the images after the first one in an image carousel may load only when the user starts to flip through the carousel.
The effect of lazy loading on SEO depends on its use. If a large piece of text containing helpful information (e.g. a Q&A section or a blog post) is lazy loaded, web crawlers may completely skip over that piece of text because it hasn’t loaded in yet — which would mean that the search engine could miss out on target keywords and understanding the structure of the page. However, if data-heavy images are lazy-loaded in a thoughtful way, it can improve page load time and create a better user experience by not overwhelming the user as soon as they load the page.