What does the "x" operator stands for in cryptography?
$\mathcal X=\mathcal M\times \mathcal R$ simply means that all elements of $\mathcal X$ have the form $(m,r)$ with $m\in\mathcal M$ and $r\in\mathcal R$. So this is an operator that takes two sets and then joins them in a tuple. This is also called Cartesian product.