Double sharp. I would almost bet a brazillion dollars (don't ask, punch line to joke) that the piece is in a minor key.
Means to play 2 half-steps higher. Example: F# means to play 1/2 step higher. Fx means to play 2 1/2 steps (or 1 whole step) or in reality a G. If you look closely there should be some little dots in amongst the X