What does '$N × Q$' represent in this relation? In relation $\{(x, y) ∈ N × Q | y = \sqrt x\}$
what does '$N × Q$' represent?

$\mathbb N$ is the set of natural numbers and $\mathbb Q$ is the set of rationals.
$\mathbb N \times \mathbb Q$ is the Cartesian product of the two sets : $\mathbb N$ and $\mathbb Q$.
Thus, $(x, y) \in \mathbb N \times \mathbb Q$ means that $(x, y)$ is an ordered pair where $x$ is a natural and $y$ is a rational.
