In the internet I have found the following definition of a Weil divisor and I am a bit confused about the definition:
For $(X,\mathcal{O}_X)$ a noetherian scheme the group of Weil divisors on $X$ is $\Bbb{Z}^{(X^1)}$ where $ X^1:\{C\subset X: C~\text{closed irreducible subset of codimension 1}\}$. More precisely a Weil divisor is a formal sum $$\sum_{C\in X^1} n_C [C]$$ where $|\{n_C\neq 0: C\in X^1\}|<\infty$.
Could someone explain me what they mean by $[C]$? So I think $n_c$ are integers but I don't get what this definition means by $[C]$.
The free abelian group on a set $S$ is the set of functions $f : S \rightarrow \mathbb{Z}$ such that $f(x) \neq 0$ for only finitely many $x \in S$. It's an abelian group under pointwise addition. The symbol $[C]$ is the function $f(s) = \begin{cases} 1 \text { if $s= C$ } \\ 0 \text{ otherwise}\end{cases}$.
Then, $\sum_i n_i [C_i]$ is just a sum of those functions.