We could not tell him the agony of our hearts, because the tongue was blistered by the fire within.

This profound couplet captures the agonizing silence of unexpressed love. The poet speaks of a situation where the sorrow of the heart was so immense that the lover could not articulate it to the beloved. The tongue, figuratively, was blistered by the internal 'fire' of emotion, making speech impossible. It's a beautiful depiction of pain that is too deep for words.