A native speaker is someone who speaks a language as his or her first language or mother tongue. Native speakers can usually speak the language very well since they speak the language since they were born. A native speaker's language is normally the language their parents speak and the language of the country they live in since birth.
It is also a term used to describe native English-speaking language instructors who teach English in non-English speaking countries. There is an organization called NativeSpeaker nativespeaker.org which promotes the development of English speaking instructors and the learning of English.