From Wikipedia, the scandle is described as "a political scandal in the United States that occurred during the second term of the Reagan administration. Between 1981 and 1986, senior administration officials secretly facilitated the illegal sale of arms to Iran, which was subject to an arms embargo at the time. The administration hoped to use the proceeds of the arms sale to fund the Contras, an anti-Sandinista rebel group in Nicaragua. Under the Boland Amendment, further funding of the Contras by legislative appropriations was prohibited by Congress, but the Reagan administration figured out a loophole by secretively using non-appropriated funds instead."
It should be noted that Iran was at war with Iraq at the time, who the US officially supported. That means Reagan was selling weapons to both sides of a war that killed at least a million people: