I first saw this in the Linux kernel style guide back in the 90s. The idea was that you should be able to read a function without scrolling. So that meant it would fit on screen in a standard terminal. It's a bit extreme for practical use, but the concept is important to keep in mind.

I was helping a Jr programmers with a code review a few months back. He'd been told that a particular chunk of code was "unclear", with specific emphasis on 4-line bit of logic in a 30-ish line function. He had no idea how he could make the code "more clear". It did what it did; the logic was a bit complex but the code was exact and correct.

The solution: factor out the 4 lines of code into a function. It was only ever called from that one place, but by making it a function you give the code a name, with clear inputs and outputs. Nothing changed about what the code did, but it became a lot easier to reason about.

It's a helpful lesson that generally doesn't really sink in for the first few years.