What does you are the music while the music lasts mean?

"You are the music while the music lasts" is poetry. There will be many possible interpretations. To me, it means "Live in this very moment, and live this moment fully".

A second way to understand this line is to explore Word meaning.

What does music represent? Joy? celebration? beauty? Next, focus on while's meaning; thus, we're faced with the fact that the "lasting" is a temporal rather than an eternal promise. To jump from T. S. Eliot to Walt Whitman is quite a leap, but Whitman's "I celebrate myself. . ." rather fits with this line. If I am the music, I choose the tempo, the lyrics and the genre.