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.
But my phone number doesn't have a dash in it!
Perhaps the site in question only allows US visitors ;). But good point! I guess there's always the danger of over-constraining possible inputs, but the general principle still holds that more guidance is better than less, and the more you can give that guidance without resorting to text the better.