OK, kiddo. Remember when we talked about the Vietnam War where the US soldiers helped the South Vietnamese fight against the North Vietnamese? Well, eventually the US soldiers had to go home and the Vietnamese people had to figure out how to take care of themselves.
So, in April 1975, the North Vietnamese army came to the capital city of South Vietnam, which was called Saigon. The North Vietnamese army was trying to take over Saigon because they wanted to unite the country under one government.
People in Saigon were very scared because they knew the North Vietnamese army was really strong and they didn't think they could possibly win against them. Many people tried to leave the city to go somewhere else where they would be safe, but there were too many people and not enough transportation to take them all away.
The South Vietnamese army was trying to fight back against the North Vietnamese army, but they were losing. Many of them gave up and surrendered to the North Vietnamese army.
On April 30th, 1975, the North Vietnamese army finally made it to the center of Saigon. They raised their flag and declared victory. The South Vietnamese government officially surrendered, and the war was over.
That's what we mean when we talk about the "fall of Saigon." It was a really important moment in history because it meant that Vietnam was finally one country again. But it was also a really scary time for the people who were living in Saigon at that time.