What does $\mathbb{R} \setminus S$ mean? What does $\mathbb{R}\setminus S$ mean? I am not getting it what it actually means. I have found it manywhere in real-analysis like in the definition of boundary points of a set. Can anyone tell me what it means really?

That symbols means set difference. It is called \setminus in $\TeX.$
If $A$ and $B$ are sets, then $A \setminus B$ is the set of elements in $A$ but not in $B$.
