First language (native language, mother tongue) is the language a person learns first. Correspondingly, the person is called a native speaker of the language. Usually a child learns the basics of their first language from their family.
Good skills in native language are essential for further learning, as native language is thought to be a base of thinking. Incomplete first language skills often make learning other languages difficult. Native language has therefore a central role in education.
The term "mother tongue" could be misleading. In some paternal societies, mothers are from different places and speak different dialects or languages. Yet their children usually only speak their local language. Only a few will learn to speak his/her mother's language like a native. Actually, mother in this context probably originated from the definition of mother as source, or origin.
One can have two (or more) native languages, thus being a native bilingual. The order in which these languages are learnt is not necessarily the order of proficiency. For instance, a French-speaking couple might have a daughter who learned French first, then English; but if she grew up in the United States, she is likely to become more proficient in English.