Speech perception is the process by which the sounds of language are heard, interpreted and understood. The study of speech perception is closely linked to the fields of phonology and phonetics in linguistics and cognitive psychology and perception in psychology. Research in speech perception seeks to understand how human listeners recognize speech sounds and use this information to understand spoken language.
Source : Wikipedia
There are (at least) six currently competing theories which seek to explain how humans manage to interpret language so efficiently - and how they seem to have the innate ability to do so.
The main theories are :
All of the above are detailed at Wikipedia