Savantism is having an extraordinary skill in a person. At least half of cases of savantism occur in Autistic people. Sometimes savant skills appear in a person after a brain injury. Other cases don't have a clear cause or association.
Current estimates of the rates of savantism in Autism are probably incorrect. Estimates are that 10% of Autistics are also savants. However, the prevalence of Autism has been underestimated for generations with many Autistics never being identified as such or being diagnosed late in life. It's likely that a savant would have raised attention and been evaluated. So, the rate is likely lower than 10%. Since Autistic people make up less than 3% of the total population, this is a very small number of people.
Examples of savant skills include: mathematical abilities such as doing calculations in one's head that require anyone else to use a calculator; master artistic abilities without training; incredible memory abilities such as being able to remember every day of one's life; calendric skills of knowing complex calculations related to the calendar such as knowing what day of the week any given day was; photographic memory, and the ability to read pages at a moment's glance. An example of a modern savant is the British architectural artist Stephen Wiltshire, who can draw an accurate cityscape from memory after seeing it once.
It's a common misconception that all Autistic people have savant skills. This is due to media presentations, most famously Rain Man, with Dustin Hoffman and Tom Cruise, which is loosely based on Kim Peek, an Autistic savant with exceptional reading, memorization, and math abilities. It's understandable why movies, television, documentaries, and news broadcasts would latch onto these stories because they are unusual and fascinating.
While we don't know what causes savantism, the likely source is neurological. Autistic brains are wired differently and injury of the brain can lead to repair that can result in wiring differences as well. Savantism was first described by J. Langdon Down who also described Down's Syndrome. It is not linked to intelligence as is often believed. Savants may have intellectual disabilities or none.
While fascinating on their own, savantism has been studied to help neuroscientists learn more about the brain. To read more about savantism, start with the Savant Syndrome Foundation.