If multiple companies talk to each other and jointly decide on such matters as the production volume and the price of a product, which should be decided by each company, is called a "cartel."

As you can see in the above illustration, if Companies A, B and C talked each other to form a cartel, there would be no competition, and the prices would be set at high levels.
Consumers would then not be able to select a product based on price, and would have to buy the product at a higher price than its original price. Since these situations would reduce consumers' benefits, this kind of cartel is prohibited as an unreasonable restraint of trade.
In a cartel, product prices are increased unreasonably, and, at the same time, inefficient companies are preserved and the economy is stagnated. For this reason, cartels are strictly regulated worldwide.