Cenizas translates to 'ashes'. It's a powerfully poetic word used to create one of the song's most memorable and melancholic images.
In the iconic line, "No le enviaré cenizas de rosas" (I won't send her ashes of roses), the word symbolizes a love that has completely burned out. This rejection of a dramatic gesture perfectly captures the song's theme of a 'light' love that was fleeting and left nothing substantial behind.
This is one of the biggest rock anthems in Latin America, and it's built on some really cool, mysterious lines. The song describes a love affair that the singer calls an "amor de música ligera" (love of light music). Maybe he means it was supposed to be simple and fun, like a pop song.
But it didn't end up being that simple. The singer admits he "nunca sorteé las trampas del amor" (never overcame the traps of love). Now, even though it's over, he says "Nada nos libra, nada más queda" (Nothing frees us, nothing else is left). It's a huge contradiction: the love was 'light', but its memory is heavy and inescapable, even when nothing tangible is left. He won't send her "cenizas de rosas" (ashes of roses), which is such a great way to say he's not going to make any big, dramatic gestures about their dead romance.