A sentence-ending construction used to give an explanation, reason, or emphasis; often appears as 'んです' in spoken Japanese. It softens or highlights the statement.
今日は休むんです。すみませんがよろしくお願いします。
I'm taking today off. Sorry, and thanks for understanding.
どうして遅刻したのですか？
Why were you late?
試験が近いのですから、もっと勉強したほうがいい。
Because the exam is near, you should study more.