This is how I explained it to my daughter (7.5):
Before: An evaluation where they will ask you a bunch of questions and the goal is to help figure out how you learn best so that we can figure out how to make school better (it was horrible that year, so this was the right explanation). The questions get harder and harder so don't worry or feel bad if you get to one that you don't know. That's the way it works and they are supposed to get harder than just about anyone will know. It doesn't mean you did badly. (Setting expectations around her perfectionism, though she did point out that on a bunch of stuff, they never did get harder and they ran out of questions -- lol, that was unexpected).
Afterward, she was with us when the tester went over the results, standard deviation and normal ranges and all. So she knew her number and seemed happy that she did 'well'.
The way I explained it to her later though was like this:
When a baby is born, their brain is like a cup. And when you are tiny, there are only a few drops of knowledge in your cup (like mama and papa love you). Over the years, you work to fill your cup up. And with hard work, you can even stretch your cup a little bit, but not forever. Your evaluation showed that you are lucky to have a very large cup. This means that it can be easier to fill sometimes and that it will hold more. But it doesn't mean you know everything yet... you still have to work to fill your cup up as much as you can by learning all kinds of new things. And having a different sized cup isn't something you can help, just like you can't help your hair color or how tall you are, so you don't want to make other people feel badly about themselves, just like you don't have to feel badly about how you are.
My DD is now 13, and she mentioned just the other day that she shared this metaphor with a group of kids who were wondering about her skipping a grade. And they all thought it was a great explanation.
