What is meant by "m|n"? Two letters separated by a vertical bar (|)

Question

I am new to this subject, and not not sure what "|" symbol means on this statement.

Let $R_2 \subset\Bbb N \times\Bbb N$ be defined by $(m, n) \in R_2$ if and only if $m|n$.

Answer 1

$ m \mid n$ means that $m$ divides $n$.

We could also say that $n$ is divisible by $m$ AND we could also say that $n$ is a multiple of $m$,so $n= a \cdot m, a \in \mathbb{Z}$.