High-functioning autism (HFA) is an informal term applied to autistic individuals. It may mean a relatively normal IQ, and the ability to speak, read, and write. HFA may simply refer to autistic people who have normal overall intelligence, i.e. are not cognitively challenged. 
"High-functioning autism" is not a real diagnostic category, but rather the impression that the child has good cognitive skills. Some studies define "high functioning" as scoring "above 70" or "above 100" on an IQ test, with no consensus about the definition. (100 is the average IQ on standardized tests of intelligence.)
Functioning labels have received plenty of criticism for being vague, oversimplifying autism, and causing potentially harmful misconceptions and stereotypes.
- High-functioning autism & Asperger syndrome
- Autistic Spectrum - A resource for High Functioning Autism and Aspergers Syndrome
- Study Provides Evidence That Autism Affects Functioning of Entire Brain: Previous View Held Autism Limited to Communication, Social Behavior, and Reasoning National Institute of Health News, Aug. 16, 2006
- Validity and Neuropsychological Characterization of Asperger Syndrome: Convergence with Nonverbal Learning Disabilities Syndrome A. Klin et al (1995) The Journal of Child Psychology and Psychiatry and Allied Disciplines, Vol. 36, No. 7, pp. 1127-1140, 1995. Reprinted with permission from Cambridge University Press. See section titled "Validity of Asperger syndrome"