The fold line is the bottom edge of what's visible when a page first loads — before anyone scrolls. It's borrowed from newspapers, where the literal fold of the paper determined what was visible on newsstands. On the web, it's more complicated because there's no single fold line. A 13" laptop, a 27" monitor, and an iPhone all have different fold lines. Your "above the fold" is someone else's "requires scrolling."
This is why designers who say "the fold doesn't matter" and marketers who obsess over it are both partially right. The fold isn't a fixed line — it's a range. On desktop, it's typically between 600px and 900px from the top. On mobile, it's between 500px and 750px depending on the device and whether the browser chrome is collapsed.
Designing for a moving target
The practical approach: design for the most common viewport first (check your analytics — for most sites it's 1366x768 or 1920x1080 on desktop, 375x812 or 390x844 on mobile). Make sure your critical content fits within the smallest common viewport. Then verify that larger viewports don't create awkward whitespace gaps.
One common mistake is placing the CTA exactly at the fold line, where it might be half-visible on certain screens — the worst of both worlds. Give yourself a buffer. If the CTA needs to be above the fold, place it high enough that it's fully visible at even the smallest common viewport size.