Answer: He enforced the Sherman Antitrust Act.
Context/history:
The Sherman Anti-Trust Act was the first measure by Congress to prohibit trusts. It was passed by Congress in 1890. A trust was when stockholders in multiple companies transferred their stock shares to a single group of trustees. Thus a whole industry area could be dominated by a single "trust" organization, destroying the free market of business competition. This was a monopolistic practice which the Sherman Anti-Trust Act ended. Thus the Sherman Anti-Trust Act directly went against the idea of those who believed business success should be based on large business owners colluding with one another.
Initially the Sherman Antitrust Act was not well enforced by US courts. But when Theodore ("Teddy") Roosevelt took office as President in 1901, he pushed enforcement of the Act and worked to reign in the power of big businesses.