An anti-dumping duty is a protectionist tariff that the government imposes on foreign imports that it believes are priced below fair market value. Dumping is a process wherein a company exports a product at a price that is significantly lower than the price it normally charges in its home (or its domestic) market.
The World Trade Organization (WTO) - an international organization that deals with the rules of trade between nations - also operates a set of international trade rules, including the international regulation of anti-dumping measures.
In the U.S., the International Trade Commission (ITC) - an independent government agency - is tasked with imposing anti-dumping duties - https://www.usitc.gov/press_room/usad.htm
For FAQ about antidumping, please visit CBP's website -