The symbol denotes whatever the author tells you it will denote in his comments about notation, and there is a special place in hell for users of unexplained notation.
It is a separator between $\forall x$ and the formula $x\in A\iff x\in B$. It means that the context of $x$ is fixed to be that of the quantified value throughout the rest of the formula.
Best Answer
This symbol is a Greek letter called (capital) sigma. It's used to denote sums. The definition is
$$ \sum_{i=1}^n a_i = a_1+\cdots+a_n. $$
For example: $$ \sum_{i=1}^6 1 = \underbrace{1+\cdots+1}_{\text{6 times}} = 6 $$
or $$ \sum_{i=1}^4 i = 1 + 2 +3+4 = 10. $$