Specifically, he was a proponent of free coinage of silver at a ratio of 16:1 (1 oz gold = 16 oz silver). The market ratio was somewhere around 30:1, so the effect would have been a currency devaluation of about 50%: this would produce a large transfer of wealth from creditors to debtors (by cutting the real value of all dollar debts in half -- this would be a major boon to households and business with high debt loads (mainly small farmers, hence Bryan's rural political base), but could be ruinous to pensioners and anyone else dependant on fixed-income assets), and in the short term would reduce both real wages and unemployment (the value of wages would be inflated down, so businesses could hire more people for the same nominal wage until prospective employees caught on).
Export industries would particularly prosper in the short term, since a pound or franc would buy twice as many dollars worth of goods, at least until US prices reacted to the devaluation. The reverse would be the case for major purchasers of imported goods.
Longer-term, there will be negative effects on foreign trade and foreign employment. The US would be on a de facto silver standard at a time when most of our trading partners were on a gold standard, so currency risk would raise the cost of trade and investment. The devaluation would also raise perceptions of regime risk of doing business with the US: if the US government devalued the currency once, what would stop the government from doing so again?