It's called sticky
Achieved using CSS position: sticky; and top: /*i.e:*/ 0px; you can find more info here:
https://developer.mozilla.org/en-US/docs/Web/CSS/position

If the element was just to move at a different pace (slower then the rest of the document scroll) than it would be called: parallax.

Sticky slideshow example:

/* Quick Reset */ * {margin:0; box-sizing: border-box;}

header,
main,
footer {
  position: relative;
  display: flex;
  justify-content: center;
  align-items: center;
}

header {min-height: 100vh; background: gold; position: sticky; top: 0; }
main   {min-height: 100vh; background: white;}
footer {min-height: 50vh;  background: grey;}

<header><h1>ABOVE THE FOLD SLIDESHOW</h1></header>
<main><div>MAIN CONTENT</div></main>
<footer><div>Footer links etc</div></footer>

It's actually position: fixed. Fixed removes the element from the normal document flow which sticky does not. For this to proberly work the scrolling part has to be "above" the top part. Which is done with z-index: int as you can see in the following screenshot:

You can read more about in the link shared by Roko.