Antitrust laws, also known as competition laws, are statutes developed in order to protect consumers through fair competition and an open market. There are a variety of business activities that may fall under antitrust laws, such as market allocation, price fixing, and bid rigging. The US government may prevent the merger of two firms or the acquisition of one firm by another if it thinks the new firm would have enough market power to create a monopoly or de facto monopoly or otherwise harm consumers.
One aspect of this regulation is that parties to significant merger transactions are required under the Hart-Scott-Rodino (HSR) Act to file notifications with the Department of Justice and Federal Trade Commission in order to allow them the opportunity to determine whether the merger might harm consumers. See also this guidance from the FTC for an overview of the premerger notification and merger review process.