"settle down" = stop what you are doing and be quiet
"Now, children, that is quite enough noise for now. Settle down and get on with your work."
OR relax :
"He settled down in front of the television with a beer."
What about = used when suggesting or offering something to someone "What about this book? Is it any good?"
In "About what", "what" is used as an interrogative expressing inquiry about the identity, nature, or value of an object or matter
"This book is about what?" Can you tell me what the subject of this book is