The doctor says that my son could have "high functioning autism"? What does that mean?

Aspbergers. It means there are symptoms of autism but not major impairments usually in communication.
Spectrum . Children with these conditions (autism, pdd, asperger's) are often referred to as "being on the spectrum." this implies a wide range (or spectrum) of functional ability. Some patients teeter on the edge of the spectrum but are still abnormal. These are the high functioning kids. It's important to diagnose so that appropriate educational & vocational interventions can be made.