What is the meaning of Believe only half of what you see and nothing that you hear?

It needs no answer, it is exactly what it is. But if you do need an answer all you have to do is think about it. What you see with your own eyes sometimes is not exactly what is happening ,but, what you "think" is happening, so, believe half of that. As for the other half of the question, believe nothing of what you hear because it is gossip, plain and simple.