Okay kiddo, I'll explain this to you in a way that you can understand.
You know how when you do something really cool, you like to show proof to other people? Like maybe you made a painting and you show it to your mom and dad or maybe you built a Lego tower and show it to your friends?
Well, when people went to the moon during the Apollo missions, they did something really cool, too. And a lot of people didn't believe that they actually went to the moon because it seemed too difficult and impossible.
So, to show everyone that they actually went to the moon, scientists and people who were watching from Earth collected something called "third-party evidence". This means that they got evidence from people who were not involved in the Apollo missions, but who were still able to see or record what happened.
For example, some people set up big telescopes to look at the moon and they could see the surface and tracks left by the astronauts. They took pictures and showed them to other people.
Another example is that people listened to the radio communications between the astronauts and mission control. They could hear the astronauts talking about what they were doing on the moon and how they were feeling. This was good evidence that they were really there, because it would be hard to fake all of that.
There were also things like moon rocks that the astronauts brought back to Earth. Scientists studied these rocks and found that they could only have come from the moon, so that was more evidence that the astronauts really did go there.
All of this third-party evidence helped to convince people who didn't believe that the Apollo missions were real. And now we know for sure that people really did go to the moon, because of all the hard work and evidence collected by so many people.