Your
Mother Tongue. The
language (in some cases, languages) which you learned
on your mother's knee. The language(s) that you knew how to speak before any formal
education commenced. The languages of which you are a
fluent native speaker, all others being
second languages.
This can change, if it happens at a young enough age. I have a friend who spoke mostly Afrikaans until he was about five. Then his family moved to an English speaking area. Now he can hardly speak Afrikaans at all.
Also means the language that you speak every day and are most fluent.
The division between first and second languages is sharpest in teaching. E.g. a English first language course will assume that you speak good English, whereas an English second language course will teach you to speak English.
Also sometimes used slightly facetiously to refer to a main programming language. Some socially awkward geeks find themselves more fluent talking in their favourite coding language to the machine than talking in their favoured human tongue to people.
E.g. if you have been writing C++ daily for 10 years, and your daily output of C++ exceeds your daily output of English, then C++ may be your first language.