silui (from sileo, silere, to be silent) means 'I was silent'.
canora is an adjective used substantivally which means melody or charm.
The English translation is a free one, perhaps in order to include the rhyme.
More literally, this is: (while) alive in the woods I was silent; now, dead, I sing melodies.
I'd guess that there's something in Ovid's Metamorphoses that's relevant. This line reminds me of a very lovely song I heard Peta Webb sing at Whitby about the violin, its wood having been part of a tree in the forest, and then, cut down and dead, able to sing as a violin.