Autism is a difference in the way a kid's brain develops. Kids with autism may have trouble understanding the world around them. They also might move in an unusual way (like flapping their hands) or do the same thing over and over (like saying the same word).
A kid with autism may have a little trouble with these things, or a lot. Some kids need only a little bit of help, and others might need a lot of help with learning and doing everyday stuff.