It's either one of the well known vocabulary of icons used across programs (e.g. floppy disk for save, house for "home", x for delete, etc), or a new one you can just hover over and read what it does in the tooltip.
After the first time (or couple of times), you don't need to read that again, and you'll recognize it very fast (not like stopping and reading it).
> x for delete
I've seen 'x', 'delete', 'del', and the trash can. 'x' is also used to cancel or close. There's little consistency.
Well, those would be bad interfaces. But the standard in OS X and Windows apps is to have tooltips.
>A further complication is that people tend to copyright icons, so the icons from one app to the next are substantially different.
Is that a thing? They might not be able to show the exact same design, but they'd still be able to show a floppy disk or trash can or whatever.
That said, OSes could (and some do) provide standard sets of icons to be used across apps, and only special app-specific actions should have custom icons.
Some people have implemented icons poorly, so we shouldn't use icons, isn't a compelling argument. Those people might implement anything badly, for instance choosing bad text in a text-based approach.
Agreed re: consistency -- good icon design requires understanding what people expect and matching it, which isn't always easy (as the referenced project demonstrates!)
Suppose I see a squiggle on some program. Where do I go to look up its meaning? I can't even type it in.
Let's take that | that now means "ON" on electrical equipment. If you guessed that it meant "1" because you're a computer/electrical engineer, wouldn't you already know what "ON" meant? If you aren't, how would you guess what | means? Flip the switch back and forth to see what it does?
If you're resorting to that, why bother marking the switch at all?