Symbolism, Imagery, Allegory
(Psst. If you haven't already read about the symbolism of the lottery, do that before your read this.)
The three caskets (gold, silver, and lead) are major symbols in the play. The big tipoff is the fact that each of them is inscribed with a message on the outside and also contains a note on the inside.
The outside of the blinged-out gold chest promises, "Who chooseth me shall gain what many men desire." Sounds nice, but it's a trick, because the inside contains a skull with a smug message: "All that glisters [glitters] is not gold [...]" (2.7.73). In other words, appearances are often deceiving, and human desire (for wealth, sex, what have you) can be dangerous.
The inscription on the outside of the silver chest reads, "Who chooseth me shall get as much as he deserves." The inside contains a picture of an "idiot," with a nasty little note: "So be gone: you are sped. / Still more fool I shall appear / By the time I linger here / With one fool's head I came to woo, / But I go away with two" (2.9.78-82). In other words, whoever chooses the silver casket is a fool who'll get what he deserves (a picture of another fool).
Finally, the lead chest, which is made of a very humble metal, seems to symbolize inner beauty and modesty (the exact opposite of the shiny gold casket) and contains a picture of Portia. The inscription is also significant: "Who chooseth me must give and hazard all he hath" (2.7.11-12).
Gee, this sounds like a pretty good description of marriage: a big risk that requires a lot of sacrifice. The inscription also reminds us of the fact that Bassanio's courtship of Portia literally involves a man who must "hazard all he hath." (That would be Antonio, who risks his life to loan his best pal the money to woo the rich heiress.)