It's the multiplier point of an extra ball. When the Multiplier reacher that point you get (or light) an extra ball.

Since it can be adjusted to a fixed or reflex value, it may always be at a set number (such as 4x) or at a moving value as the machine tries to keep the extra balls at a set percentage (say 10% or 15%).

It's all in the adjustments and in the manual...

edit to add: Reading the instruction card on the machine (the one right there on the apron) says that the flashing X awards a special.

So It really is a special (that could award an extra ball).

Adjustments: 07 special awards points/ex ball/credit
45 Special Auto Adjustment (moves the special award to try to hit percentage)
46 Special @ X (set to 2x to 7x), If 45 Auto is on this is the start point.

_{Post edited by themadman : more info}