In reading piano music, what does an X mean in the treble clef?

Answer 1

It's a double sharp. It means you have to sharp the note twice. So it usually makes a white key sharp, not to the next black key, but to the next white key.

Answer 2

It is a double sharp, placed to the left of the note. Raises the note by two semi-tones. I've always wondered why it's necessary!