proving that $P$ is a partition

On a practice exam, our teacher gave us this answer as the third point in proving:

Let $n$ be a positive integer and let $P = \{$equivalence classes for is-congruent-to-mod-$n\}$. Show that $P$ is a partition of the set of integers.

$\bigcup_{a\in\mathbb{Z}_n} [a] = \mathbb{Z}$: Clearly $\bigcup_{a\in\mathbb{Z}_n} [a] \subseteq \mathbb{Z}$ as each element in each set is an integer. Now let $z \in\mathbb{Z}$. By the division algorithm there is a unique pair $(q,r)$ with $0 \leq r < n$ such that $z=qn+r$. Thus $z\equiv r\pmod{n}$. That is $z\in[r]$, so $z\in\bigcup_{a\in\mathbb{Z}_n}[a]$.

My question is, I do not understand what this is saying. I do not understand the notation, nor do I understand what this is proving.

2022-07-25 20:41:00
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