What means a "$\setminus$" logic symbol?

Solution 1:

"$\,A\setminus B\;$" $\,$ means the $A$ "set minus" $B$: that is, it defines a region which contains all points $(x, y)$ such that $\,(x, y) \in A,\,$ but $\,(x, y) \notin B$.

$$A\setminus B = \{ (x, y)\mid (x, y) \in A \land (x, y) \notin B\}.$$

In your case, the region $A\setminus B$ means "all points satisfying the system of inequalities defining set $A$, but not satisfying the system of inequalities defining set $B$.

Solution 2:

This is the set difference operator. $A\setminus B=\{a\in A\mid a\notin B\}$.

For example $\{0,1\}\setminus\{1\}=\{0\}$. And $\Bbb Z\setminus\Bbb N$ is the set of negative numbers. $\Bbb R\setminus\Bbb Q$ is the set of irrational numbers, and so on.