Antitrust laws are the laws of competition, which were set to check the abuses made by the Big Trusts and monopolies in the 19th century.
The firsts laws against monopolistic practices were "The Interstate Commerce Act of 1887" and "The Sherman Anti-Trust Act of 1890".
1 example of an antitrust effort could have been dropping drastically the price of a product or service only in certain area despite losing profit, if another company enters the business with a slightly better price, in order to eliminate that competition. So the big company choose to lose money in that area solely for the purpose of expel the competitor and put them out of business.
Another example is the collusion between several companies to set an specific same price on certain item they sell separately, in order to control the market.
A key example: The Sherman Anti-Trust Act
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.