This is the 2803rd most frequent German word.
reine
"Reine" in German means "pure" or "clean" in English.
Here 'reine' is used as an adjective to emphasize the purity or simplicity of 'time waste'.
Das ist reine Zeitverschwendung.
That is pure waste of time.
In this case, 'reinem' (a declined form of 'rein') describes the gold metaphorically as flawless or perfect.
Seine Stimme war von reinem Gold.
His voice was of pure gold.
Here, 'reinem' modifies 'Herzensgut', signifying the untainted, genuine quality of her action stemming from goodness.