I Wonder What My 'Fence' Is?

That's a great question, which I haven't seen anyone else ask.
If you're talking about personal fences, I'd say you could just look to the things you know you shouldn't do - whether morally, or as decreed by law, your community, your spouse - for those rules and injunctions, written or unwritten, are in a sense walls. Lines beyond which you know not to pass. You might question the historical origins of those laws and codes, as well as whom they purport to protect versus actually benefit, and questioning what it is exactly that you fear in potentially transgressing them.

If you care much for philosophy and theory, there is a large body of literature regarding the ways in which reality is always mediated; whether by the structures of language, social hierarchies and apparatuses of power, or by consciousness, or the functions of the brain itself. 20th century psychoanalysis has a lot to say in this area; less Freud and more people like Jacques Lacan and, later, Slavoj Zizek (both are difficult, but Zizek could be more fun). Even Marx, if you aren't opposed to reading him, shows how our very ability to have certain thoughts is mediated by the structure and logic of capital.

If you want a movie that approaches these ideas in a somewhat interesting way, yet is accessible, try John Carpenter's 'They Live' from 1988 (strange as that recommendation may seem just give it a shot).