This statement suggests a wise approach to communication:
"If you cannot be positive": It acknowledges that positivity isn't always possible in every situation.
"Then at least be quiet": This part emphasizes that in times when one cannot contribute positively, it's better to remain silent.
In essence, this statement encourages restraint in speech when one cannot express positivity or contribute constructively to a conversation. It underscores the value of avoiding negative or unproductive words in order to maintain a more harmonious and peaceful environment.