Computing epsilon-closure. What does $E(n) \leftarrow \{n\};$ and $E(p)$ mean?
For each state $n \in N$, we maintain a set of states $E(n)$. Eventually, $E(n)$ would contain all states reachable from $n$ via $\epsilon$-transitions.

You can think of $E$ as an array of sets. The notation $E(n)$ just means the value of $E$ at position $n$. So $E(p)$ is just the value of $E$ at position $p$.