Unknown syntax with multiplication?

It means "the product over all pairs $i, j$ where $i < j$." For example, if $p(i)$ is defined for $i = 1, 2, 3$, then $$ \prod_{i<j} \;[\,p(j) - p(i)\;] = [ p(2) - p(1) ] [p(3) - p(1)] [p(3) - p(2)]. $$

It means the product of every version of the expression in which $i <j $ and $i $ and $j $ are in some set implied from context.