A polite explanatory or emphatic auxiliary used to give reasons, soften statements, or assert something; often contracted from 'のです' and commonly heard as 'んです'.
どうして遅れたんですか。
Why were you late?
これは私の本なんです。
This is my book (that's why I'm keeping it).
実は、来週引っ越すんです。
Actually, I'm moving next week.
彼が犯人なんです。信じられないでしょうが。
He's the culprit. You probably can't believe it, I know.