They're just talking about affordance[1], but it's a pretty good example. Anyone interested in this might also like the book The Design of Everyday Things.
Thanks! I loved that book when I read it a few months ago. One of the best examples they give of this concept is a VCR. There's only one way to put a tape into a VCR, and it isn't done with instructions or symbols, it's done using affordance or physical constraints. It doesn't fit any way but the right way.