$$ \iint_S x+y+z \,dσ $$
$$ 0 \leq x,y,z \leq 1 $$
I have this surface integral where $S$ is the surface of a cube. How do I find the parameters $x,y$ and $z$? Also,what does the second inequality means? Does it means that the values of $x,y$ and $z$ are between $0$ and $1$? I have read the chapter of surface integrals but I didn't understand a lot from the theories. Is it about a function in space $f(x,y,z)$ ?