what does the second treble clef mean above the normal one when you play the piano?

Answer 1

On sheet music the upper treble is the main melody.

It is usually the part the voice sings, and the notation is usually simplistic.
You can see this esp. with sheet music for popular songs.

Answer 2

Does this clef have it's own staff? If so, then it's an optional harmony part, probably meant to be played either an octave above where it's written, or on another instrument (there's a word for it...I can't think of what it is. Argg!)

If, however, it's just planted on the main Treble clef like a hat, then this is an archaic way of telling you to play an octave above where written (I think). Most modern music just writes 8va there.

Answer 3

Probably that the notes are so high a second clef was need to notate them. Or it is for a second player, like a duet.