Operation Merlin was a fancy name for a secret plan that the United States government made to trick another country called Iran. The United States government wanted to make Iran think they had gotten information about how to make a nuclear bomb, but the information was really fake.
Imagine playing a pretend game with your friends. One friend pretends to have a secret, and then you pretend to try to get the secret from them. Operation Merlin was kind of like that. The United States pretended to give Iran some secret information, but it was actually a fake secret.
The reason they did this was that they were worried that Iran was trying to make a nuclear bomb. A nuclear bomb is a very dangerous weapon that can cause a lot of destruction, so the United States wanted to make sure Iran didn't make one.
That's why the United States made a plan to trick Iran into thinking they had the secret, but really they were just giving them fake information. This way, Iran wouldn't know the real secrets of how to make a nuclear bomb, and the world would be safer.
In the end, it's not entirely clear whether Operation Merlin worked or not, but it was a very important part of the United States' efforts to keep the world safe from nuclear weapons.