Notation question:

What does $(\Delta^1)^{1,...,n-1}$ denote? Note here: The 1,...,n-1 should be enclosed in curly braces, but it appears to be a limitation of the jsmath application to get those to work like they do in LaTeX.

Vocabulary question:

Suppose $z:\Delta^{n+1} \rightarrow S$ is a morphism of simplicial sets. What does the following translate to in algebraic terms: $z|\Delta^{0,...,n}$ is a constant simplex at a vertex $x$. Note again the curly brace issue.

So mainly, I just don't know what that is supposed to mean, "is a constant simplex at the vertex x". Everything else makes fine sense.

I've searched through a number of books on homotopy theory, algebraic topology, etc. and I've been unable to find these precise usages.

I ask these questions only because I'm reading HTT by Lurie, and these usages come up and they're quite confusing.