What do $0^n$ and $1^n$ mean in cryptography? Let $f$ = $\{f_k\}$ be a pseudorandom function family.
Let $G(x)$ be a pseudo-random generator such that:
$G(x) = f_x(0^k)f_x(1^k)$ where $k=|x|$.

I don't understand the meaning of $1^n$  and $0^n$, and the differences between them, in that context.
What do they represent?
What is the special role / effect they have on the above context? And how?
Why $1^n$ and $0^n$, and not other combinations?
Typically that means a string of either $n$ zeros or $n$ ones.