わかりやすい解説。

script that allows class toggle when object enters viewport, to trigger transitions or animations. Simplest most basic would be fade in on entering window but allows for more sophisticated use that I wouldn't be able to do until I learn more js.

You'd expect that button in the graphic to be visible (assuming this element is at the top of the page and you haven't scrolled) since it's along the bottom edge of a 100vh element. But it's actually hidden behind the browser chrome in mobile browsers, including iOS Safari or Android Chrome.

about the 100vh problem on mobiles (it's calculated without including address bar height)

100vh on mobiles

Explains that 100vh on mobile sets vh to height of window with address panel hidden, so that when address bar is visible then window is smaller but vh won't update itself.
See comments at bottom for workarounds

The core issue is that mobile browsers (I’m looking at you, Chrome and Safari) have a “helpful” feature where the address bar is sometimes visible and sometimes hidden, changing the visible size of the viewport. Rather than adjusting the height of 100vh to be the visible portion of the screen as the viewport height changes, these browsers instead have 100vh set to the height of the browser with address the address bar hidden. The result is that the bottom portion of the screen will be cut off when the address bar is visible, thus defeating the purpose of 100vh to begin with.