At their core, antitrust laws are meant to break up businesses who work together to act like monopolies. Monopolies reduce the amount of competition in the market place, as this term indicates when one company has cornered a share of the market. For example, if only one company made cellphones, this would reduce competition and increase the price of phones since you can only them from one place.
By implementing antitrust laws, the government is aiming to make sure that there are no monopoly like coalitions. Monopolies have a negative effect on the consumers, as they can manipulate prices.