Maybe is referring to the space of Lipschitz functions where the boundary of the space is in this set?
The title says it, I can't find what is a Lipschitz space, my analysis teacher mentioned it the other day but he said it wasn't important for now. I think it has something to do with the fact that the boarder of the space can be seen as Lipschitz functions. But this isn't a very good definition.
Thanks in advance!