If you opened a sealed envelope marked "Confidential" without permission, it could be considered as an unlawful opening of a letter and you could be punished with imprisonment with work detail for under one year, or a fine of up to 200,000 yen.
Your parents don't want this to expand out into the community. Let's keep this confidential.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.