You could read the whole story in 10 min. And be done with it. But this movie stretched it into almost 2 hours long and boring - whatever it is.
Tl;dr The guy shorts the stock (makes a bet). Another one doesn't like this guy so he buys the stock (makes a counter bet - longs the stock). Others join the second one. The second one wins. In the end the first one was right, but didn't make money out of it, because he has a punchable face and nobody likes him and stock isn't about being right or wrong but about gambling and how good of a gamler are you. The first one isn't a good gambler.
Tl;dr The guy shorts the stock (makes a bet). Another one doesn't like this guy so he buys the stock (makes a counter bet - longs the stock). Others join the second one. The second one wins. In the end the first one was right, but didn't make money out of it, because he has a punchable face and nobody likes him and stock isn't about being right or wrong but about gambling and how good of a gamler are you. The first one isn't a good gambler.