To provide a detailed analysis, I'd need to know which story you're referring to, as many narratives use the motif of a "beating heart" symbolically. However, in many literary contexts, the beating heart often represents themes such as guilt, conscience, life and death, the passage of time, or an emotional core that drives the character's actions.
If you're discussing something like Edgar Allan Poe's "The Tell-Tale Heart," for example, the beating heart symbolizes the narrator's overwhelming guilt and descent into madness. It serves as a reminder of his crime, ultimately leading to his confession.
Could you specify the story you are referencing so I can provide a more tailored response?