The phrase meant that Oberon’s plan through Puck to be-spell Titania into falling in love with an ass had succeeded.

Oberon, King of the fairies, was dejected at what he claimed was Titania, Queen of the fairies, refusal to allow him to have the boy who was stolen from the Indian king. Oberon was jealous of the boy because of all the attention Titania directed towards him, which also estranged the relationship between her and Oberon.

To torment Titania, Oberon commissioned Puck to obtain the juice from a particular flower so he could apply it on the sleeping eyes of Titania. The expected effect was that, Titania would fall madly in love with the first live creature she would set her eyes on when she woke up. To reverse the effect Oberon would then ask for the boy in exchange.

Once I have this juice,
I'll find Titania when she is asleep,
And drop it into her eyes.
The next thing she looks upon, when she wakes up —
Whether it’s a lion, bear, or wolf, or bull,
Or meddling monkey or a busy ape —
She shall pursue that animal with the soul of love.
And before I remove this charm from her sight —
Because I have another herb that is the antidote —
I'll make her give that boy to me. (Oberon)

Oberon applied the juice on Titania’s eyes and Puck changed Bottom’s head into an ass’s head. When she woke up, the first thing she saw was Bottom in the ass’s head and instantly fell in love with him.

My ear is very much in love with your song.
So my eye is very attracted to your shape;
And your fair virtue's force violently moves me,
On the first look of you, to say, to swear, I love you. (Titania)