What It Is
Above the fold is whatever a visitor sees the moment a page opens, before they scroll. The term comes from newspapers. A broadsheet was sold folded in half, and only the top half showed on the newsstand, so editors put the biggest headline and the best photo there. On a website the fold is the bottom edge of the screen, and everything below it has to be scrolled to.
How It Works
There is no single fold. Where it falls depends on the screen: a wide computer monitor shows a good deal of a page at once, and a phone shows much less. Since so many local searches happen on a phone, the fold that matters most for a local business is the bottom of a phone screen, and what sits above it is the first screen a customer sees after tapping your listing.
Only a few things belong there. A headline that says what the business does and where, so a visitor knows at a glance they have landed in the right place. The one thing you want them to do, the page's call to action, such as a Call button or a booking button. A large photo or a slider that fills the whole first screen pushes both below the fold, and a visitor sees a picture with nothing to do. None of this means a page should be short. Visitors scroll when the first screen gives them a reason to.
Why It Matters for Being Found
The first screen decides whether a visitor stays. Someone who tapped your listing from a search makes up their mind quickly about whether they found the right business, and if the first screen does not tell them, they go back to the results and tap the next one. Nothing further down the page gets a chance to change their mind. It also shapes how fast the page feels. The biggest thing above the fold is usually what Largest Contentful Paint times, so a heavy photo there is both the first thing a visitor waits for and part of how Google measures the page.