Men in Black are characters that you may have seen in movies or TV shows. They are usually seen wearing black suits, sunglasses, and hats. They are called "men in black" because of their outfit.
Men in black are often portrayed as secret agents who protect the Earth from aliens. However, they are not real people. They are fictional characters that were created to entertain us.
In real life, there are government agents who work for agencies such as the FBI, CIA, or Homeland Security that may wear dark suits and sunglasses, but they don't have the nickname "men in black."
So, when you hear the term "men in black," remember that it's just something made up for movies and TV shows.