Example
Two roommates are home on a Saturday afternoon. Roommate 1 is tired and wants to take a nap. Roommate 2 is playing video games in the living room.
Roommate 1: Hey, I'm going to go take a nap. I'm so exhausted. I didn't get home from work until 3 am this morning.
Roommate 2: Sure, no worries! Rest up.
Roommate 3 comes home and joins Roommate 2 in playing video games. They get really involved in the game and start shouting and turn up the volume. Roommate 1 is awakened and upset by all this noise.
Roommate 1: Would you guys pipe down in there! I'm trying to sleep!
Roommate 3: Sorry! We'll be more quiet.
Explanation
Pipe down is used the same way as "be quiet." This expression is used as an imperative, or a command. It is commonly used in anger or urgency. Pipe down is an in-separable phrasal verb.
More examples:
Pipe down! I'm trying to study!
You kids need to pipe down! The baby is sleeping!
