Symbolism, Imagery, Allegory
What's the matter with us? Nuh-uh, what's the motif with you? Sorry, we're reaching here, we know. Anyways, a motif is basically a recurring symbol or image or whatever. And cages, chains, and graves—oh my—definitely recur throughout Thérèse Raquin.
Early in the novel, Thérèse feels as if she's being buried alive in the shop… because she's got a feisty personality, and can't stand her unexciting life. This mismatch is what leads her into the arms of Laurent—the seductive artist who offers her an escape from her everyday life.
After Camille's death, Laurent and Thérèse are plagued by Camille's ghost—so they feel trapped in their very own bedroom. And the more the two feel chained together, the more desperate they become to break free. Which prods them to try to kill each other.
Zola's emphasis on how people have no free will—they're just products of their biology and environments, a.k.a. scientific determinism—also emerges in this motif. Metaphorical cages are what directly (and inevitably) cause the book's greatest tragedies.