This is the 944th most frequent Polish word.
piękny
Beautiful.
The word 'piękny' is used as a descriptive adjective to express aesthetic admiration.
Ten obraz jest piękny.
This painting is beautiful.
'Piękny' here describes the favorable and pleasant quality of the day’s weather in nominative form to match the direct object.
Piękną pogodę mamy dzisiaj.
We have beautiful weather today.
In this context, 'piękny' is expressing admiration for the quality of her voice, positioned attributively.