This claim is a statement of the philosophy of idealism.
According to idealism, whenever we say that some object has some quality, what we are really talking about is how we perceive that object. So, if I say that fire is hot, what I am really saying is that when I get close to a fire I perceive its heat. Idealists would argue that this quality of fire does not really exist if no one is perceiving it.
Therefore, anything that exists is perceived. Perception can be equated with existence. A thing only exists if it is perceived. Therefore, it is accurate to say that “to be is to be perceived.”