The line is from Act 2, sc. 1 and it is spoken by Oberon to Puck. He is asking Puck if he remembers the time he heard a mermaid who was riding on a dolphin and singing with such sweetness that her song calmed a rough sea and caused stars to fall from the sky just to hear her. He goes on to tell Puck that it was this same night that he saw Cupid shoot his arrows at a young virgin, but the arrows missed. The arrows, Oberon continues, hit a white flower which caused the flower to turn purple as it was now filled with love. It is this flower which Oberon sends Puck to find so that the nectar can be applied to Titania's eyes to make her fall in love with whatever she sees first upon awakening. It is also this flower and its nectar which creates the humor and confusion among the four lovers: Helena, Hermia, Lysander, and Demetrius. This occurs when Puck accidentally annoints the wrong young man.