What does this symbol mean? (looks like a 1 with double vertical line)
Solution 1:
It's the characteristic function (or indicator function) of the set in the subscript.
$$\mathbb 1_A(x) = \begin{cases} 1\,, & x\in A \\ 0\,, & x\notin A\end{cases}.$$
Solution 2:
It is the indicator function.