also: なのです
auxiliary
it is that ...; the thing is ...
Polite explanatory/assertive auxiliary attached to nouns and na-adjectives. Adds a tone of explanation, emphasis, or seeking agreement. Equivalent to のです with politeness.
See also: のです
これは私の本なんです。
This is my book (you see).
彼は学生なんです。
He is a student (that's the situation).
Contraction of なのです, from な (attributive form of copula だ) + の (explanatory particle) + です (polite copula).