leave things as they are. When you go to someones house and they are like leave things as they are, and don't touch anything, that's what it means.
It means not to bring up things from the past that are best left alone, so as not to cause trouble.
It's an old English idiom. The original author is unknown.
let sleeping dogs lie means let someone sleep someone can be anyone like your neighbour
It means don't stir up trouble.