A polite past-negative form attached to the -masu stem of a verb; used to say that someone did not do something (in the past) in a polite manner.
昨日は雨が降りませんでした。
It didn't rain yesterday.
昨日の会議には参加しませんでした。
I did not attend yesterday's meeting.
先週は宿題を出しませんでした。
I didn't turn in the homework last week.