This is the 6026th most frequent Chinese word.
安靜
"Quiet" or "peaceful."
Here, "安静" is used as an adverb modifying the verb "坐下" (sit down).
安静地坐下，我们开始上课了。
Sit quietly, we are starting the class.
Here, "安静" is an adjective describing the state of the room.
房间里很安静，只有时钟的滴答声。
The room is very quiet, with only the ticking of the clock.
In this sentence, "安静" is used as a command asking for silence, functioning as a noun.