A contemporary rendering of that passage from "Twelfth Night" by William Shakespeare might be:
ORSINO: If music is the sustenance of love, keep playing,
Give me an abundance of it; let me indulge
Until the desire becomes nauseated and fades away.
(Act I, Scene 1)
In this passage from "Twelfth Night," Orsino, the Duke of Illyria, expresses his feelings about the power of music in relation to love. Let's break down the passage:
"If music be the food of love, play on,"
Orsino begins by making a metaphorical comparison between music and the nourishment (food) of love. He's suggesting that just as food sustains the body, music sustains love.
"Give me excess of it; that, surfeiting
The appetite may sicken, and so die."
Orsino goes on to express a desire for an abundance of music. He wants to hear so much music that it becomes overwhelming, causing his appetite for love to become "surfeited" or excessively filled. The idea is that by indulging to the point of excess, he hopes to make his feelings of love subside or diminish, perhaps because he is experiencing unrequited love or is troubled by the intensity of his emotions.
So, in more straightforward terms, Orsino is saying that he believes music has the power to influence and perhaps even alleviate the pain of love, and he asks for an excess of it in an attempt to cope with or diminish his feelings.