Above the fold is whatever a visitor sees on screen the instant a page loads, before they scroll at all, a term borrowed from newspapers, where the fold literally determined what showed on a newsstand. On the web, it's not a fixed pixel measurement; it varies by screen size, device, and browser, which is exactly why designing rigidly around one fixed "fold line" is less useful than it sounds.
It carries real weight because it's a visitor's first and sometimes only impression: if the content, value proposition, or answer to their query isn't at least suggested within that first screen, plenty of visitors leave before ever scrolling to find out what's below. That's also why above-the-fold content is closely tied to Largest Contentful Paint specifically, since whatever's visible immediately needs to actually render fast, a slow-loading hero image or render-blocking script sitting above the fold delays the exact content most responsible for whether a visitor sticks around. Good practice keeps the space focused: a clear indication of what the page offers and enough of a hint to earn a scroll, rather than trying to cram an entire pitch into the first screen.
