They mean like you walk up to a public stall, you see it's empty (because it's see through glass), so you know it's safe to go in. You lock the door when you go in and it turns opaque so nobody can see you pooping.

Edit: I found a video of what they are talking about. Apparently it's also so that you know the bathroom is clean and not gross so you aren't discouraged from using it. Pretty neat. https://youtu.be/KGoKsv2omBU?si=F0ig6oFHtavzZY9z