The Apple Box, as it's known in the grip department is an extremely versatile tool used in film and television set production. It's a wooden box, with handles, and comes in a variety of sizes.
The large size, or Full Apple, is uniformly 20" x 12" x 8". The size and term come from the old-fashioned produce crate that you might find at a grocery store, apple orchard or your favorite farmer's market.
The next size is a Half Apple (20" x 12" x 4"), then Quarter Apple (20" x 12" x 2") and then the Pancake or Eighth Apple (20" x 12" x 2").
These sturdy boxes are used for propping up items, often temporarily, and even as chairs or lifting stands for actors and also crew. Sometimes they're needed to help elevate dolly tracks or actors. Sometimes they are referred to as "man makers", but often not in front of actors who are shorter than their leading ladies.
There are also Mini Apple Boxes, which are 10" in length rather than 20", and sometimes you'll find a grip department that uses nesting Apple Boxes as well, but their structural integrity isn't nearly as strong.
To read more, Sources: Uva, Michael. The Grip Book (Fourth ed.)