I don't think interface was ever actually a term in our reading, but it has come up in class enough times. As far as I understand it, an interface is a sort of expected set-up for a viewer to use or navigate information. This understanding comes, in part, from one time when I heard the professor tell one of my classmates that he was folding his brochure wrong, and she said, "That's not the proper interface!" And now that I've typed that out, it doesn't quite make the connection it makes in my head, so you'll just have to trust me on this one, Imaginary Reader.
So, assuming I understand interface properly, it's an expected sort of system that helps you get around. Let's get back to my road signs, shall we? When you see a red octagon with white letters, you probably assume you should stop. (Unless you're from Europe, where road signs are different than they are here in the US, and then I don't know what your stop sign looks like.) If you see a little white rectangle with a red border on the side of the road, your first thought might be that it's a No Parking sign. If it's a bigger white rectangle with a black border, it's probably a Speed Limit sign, and you ignore it unless you're a goody-two-shoes like me, in which case everyone passes you while you mutter at them for being too good for the speed limit.
This predictable system of roadsigns, the same all across America, is one big interface. From Alabama to Alaska, if you're on the road and you see an orange sign to the right, you know there's construction. If a stoplight turns red, you stop. (Or, you go really fast and hope you don't cause an accident.) If you've been raised with it, some of it seems almost intuitive, but really you've just been indoctrinated with it. This is the interface you expect, and if you woke up one day and all the roadsigns across the country had suddenly switched around, so that red borders were speed limits and black borders were No Parking, and orange signs were landmarks and brown signs were construction, then there would be a lot of confused drivers on the road.
Even slight variations in this interface can throw people off--or at least, I presume that's what happened in the case I saw while walking on the trail. I passed a sign alerting the trail's users that there was an intersection ahead--a three-way intersection, that is. Apparently, someone was so accustomed to four-way intersections that they painted the fourth direction onto the sign, because it just wasn't acceptable to have a sideways-T where there should have been a plus sign.
Of course, it was probably just someone being funny, but I still wish I'd taken a picture.