The jungle represents the modern, cut-throat world of business competition in which some, like Ben, might triumph while others, like Willy, go under. It is a jungle in the sense that people are almost literally fighting each other to get to the top; it suggests a ruthless world where only material success matters, and the weak, or unlucky, go to the wall.
Ben and Willy, two brothers, represent the two ends of the spectrum. Ben (who appears to be considerably Willy's senior) took the bold move of heading out to Alaska and ended up a very wealthy man: 'By God, I was rich!' as Willy remembers him saying. Because of this mercantile success, Willy simply idolizes him, looking up to him as a role model to follow. Another role model for Willy is the old salesman Dave Singleman, whom we learn about in one of Willy's frequent reminiscences and who apparently was still effortlessly making money well into his eighties. Willy strives desperately to be like such individuals, but he never strikes it big and ends up killing himself.
To the last, however, Willy remains obsessed with his material dreams, imagining that his son Biff can get ahead in the business world with the insurance payout after his suicide. Biff, however, is under no illusions about the business world - 'screw the business world!' he exclaims angrily at one point. After a string of failures, he finally determines to get out of the modern 'jungle'.
The jungle symbolizes the wild world of commerce and business where anything can happen, and one can strike it rich. Ben says to Willy: "the jungle is dark but full of diamonds," meaning that it's chaotic and difficult out there, but you can make it. Willy never does make it, never does find the diamonds. This is especially hard on him as he believes it is possible to make it in "the jungle"