This is what we call a " discourse marker", meaning it is an expression we use to tell the listener or reader that we're going to introduce additional information after we have already given at least one point. It means,"And in addition to (X which was just mentioned), ....",
e.g.,
1)"I don't really like the latest marketing plan. What's more, it's going to be very expensive."
2) "The Thai delegates will be arriving early in the day. What's more, they have all changed their hotel reservations."
"What's more" is like other expressions for adding points: "moreover", "in addition", "furthermore", etc.