Fence, Hedge
A fence, a hedge, or a barrier obviously symbolizes some form of boundary that is typically possible to pass, a minor obstacle (which may be more of a transgression than an impossibility to get past); let’s say a relatively enclosed or even framed part of our inner self.
Since regular fences are so easy to get past, they are usually a symbolic marker, indicating that the owner does not want one to pass without invitation. It signals: "I want to be left alone inside here and choose for myself who I meet on my land."