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.