Who is William Jennings Bryan from The Rise of Theodore Roosevelt and what is their importance?

Twice, William McKinley runs for president against Williams Jennings Bryan, a liberal democrat whose strongest platform is that of a silver standard. He purports that such a monetary standard would give more money to the poor. Roosevelt argues for continuing the gold standard. He demonstrates that money on a silver standard buys less food than money backed by the gold standard. Bryan always gives in to McKinley's victories gracefully, citing that the American people speak on the issue, with their votes.