Symbolism, Imagery, Allegory
"Good fences makes good neighbors," penned Robert Frost. (By the way, if you and Robert Frost are not friends, please get to know him a little better. You can thank us later.) But what Frost didn't know is that zombies are never good neighbors. Fences or not, sharing space with creatures with rotted faces and nasty death-breath will never be awesome.
The fences do provide some safety, though, and to most of the villagers, they are a source of relief and hope. Why? Because with strong fences, folks can live in relative peace and safety. Sure, there are moaning flesh-eaters beyond the chain links, but inside of them, villagers know they can still marry, raise kids, and avoid certain death and zombiehood.
As always, Mary's the odd duck in this regard. While the rest of the villagers waddle around feeling safe and sound, she sees the fences as a chain-link prison cell. Within them, she must live in fear and helplessness.
This doesn't end with Mary's journey on the paths, though. Despite escaping the fence-like trap of the Sisterhood and their rules for the village society, Mary still feels trapped in the world in general. Stupid fences.
Ironically, the only way Mary escapes the terror and entrapment of the fences is by slipping through the gate to become "the one on the other side of the fence" (34.19). Wait—what? Yup—by finding the courage to get past the fences, Mary finds her precious ocean and the end of entrapment as she knows it.
But don't worry. There are still zombies at the ocean too, so she'll still be stuck right back behind some sort of fence line to escape them. But that's a story for another book.