The headstone is a symbol of friendship, caring, and compassion. Headstones themselves are symbols – they’re a sign of respect for the person buried on the spot. They’re meant to keep that person's memory alive in the world of the living. A headstone says, “I am remembered” and "I am worth remembering."
But Liza doesn't have a headstone. We can understand why she's upset about this. Because she was accused of being a witch, she wasn't allowed a proper burial. She has nothing to mark her spot. It takes Bod, a live boy with a huge heart, to notice this unfairness and make things right. We might also look at her headstone as a symbol of our ability in the present to address and fix unfairness in the past.