Okay kiddo, so there's this country called Iran which is a really big place with lots of people living there. Some of these people are really smart and have a lot of knowledge and skills in things like science, technology, and medicine.
But sometimes these smart people decide to leave Iran and go live in other countries. This is called a "brain drain" because it means that Iran is losing some of its smartest and most talented people.
Now you might wonder, "Why would they do that?" Well, sometimes people leave their home country because they can't find good jobs or because they feel like they have better opportunities in other places. Sometimes they also leave because they don't agree with the government or because they feel like they're not free to say or do what they want.
The problem is that when lots of smart people leave a country, it can make it harder for that country to grow and develop. It might be harder to solve problems or come up with new ideas because there aren't as many talented people around to help.
So the brain drain is a big deal for Iran, because it means they're losing some of their most talented and educated people. And that can make it harder for the country to keep up with other places in the world.