| Portia sings this song to Bassanio as he is about to choose a casket that will determine if they are to be allowed to wed or not. Previous suitors had chosen the gold and silver caskets and lost their chance, for Portia's father had placed her portrait in the leaden casket, assuming no one would pick lowly lead.
One of my english teachers suggests that this song is Portia's way of dropping hints to Bassanio, the suitor she prefers. Notice how bred, head, nourishèd, fed all rhyme with lead, and the emphasis on the bell. (Jonathan Miller's 1973 TV adaptation (the cool one with Laurence Olivier in it) portrays this to excellent effect, with Portia and her attendants dressing all in grey, and heavily emphasizing all the words that rhyme with lead.) More subtly, I think it is a warning to Bassanio to beware of superficial engagement and to look beyond deceiving exteriors.
Armed with these clues, Bassanio correctly chooses the lead casket and wins Portia's hand. | 
