The symbol is logical equality. The expression "LHS RHS" indicates that (a) the left hand side (i.e. LHS) and right hand side (i.e. RHS) are true, or else (b) the left hand side and right hand side are false; there is option (c) (e.g. it is possible for the LHS statement to be true and also simultaneously for the RHS statement to false), because otherwise "LHS RHS" would not have been written (see this footnote for an example illustrating this concept).