How little? What kind of resources do you have to explain it?
If you're doing a presentation to a lot of kids, maybe start by saying that Graphic Designers are problem solvers. What kinds of problems do they solve? Well, let's say that Jimmy has a lemonade stand, and needs a sign to post at the end of his street, so people will come down and buy his lemonade. That's the problem. "How to attract people to the lemonade stand"
Then talk about the basics of design.
1) Information. What words need to be on the poster? What he's selling, where the lemonade stand is, how much the lemonade costs.
If Jimmy puts all those words really tiny on a poster, so nobody can read it... that's bad design because people won't be attracted if they don't get the information. Maybe even show an example of what a poster like that would look like.
A designer's job is to decide which information is important and make sure someone who looks at the poster sees the most important thing first. COLD LEMONADE might be the biggest words, and the price will be second-biggest, and the location will be third biggest.
After showing the poorly designed poster and the well-designed poster side by side... show other types of graphic design. Movie posters or advertisements from magazines. Have the kids point out which information is the most important, or guess what problem the poster or advertisement was trying to solve.
Then maybe introduce them to things they've never thought about. The movie poster for UP has the words UP, but they look different, like you're looking up at the letters. Show the kids and ask them if they notice anything about the letters. Once they notice, tell them that it was a graphic designer's idea to make the word UP look that way.
Show them the poster for A Bugs Life or Cars or other pixar movies they might already know. The logos all say something about the movie. Show them what A Bugs Life looks like in a plain old Times New Roman font, next to the logo, and ask the kids which one tells them more about the movie.
I know this is a lot, but your question was kind of vague haha. Hopefully there's something in here you can take away to help explain.