In Poe's short story "The Tell-Tale Heart," the narrator murders an old man he lives with, dismembers the body, and buries it under the floor. As the narrator contemplates the murder, standing in the doorway of the man's room, he believes he hears the beating of the man's heart as he lies in his bed. The narrator believes that due to an illness his senses have been heightened. He believes the man's heart is beating loudly in fear. As he descends upon the man to suffocate him, he continues to hear the beating, but at last the sound subsides, and the narrator determines the man is dead.
After the narrator buries the body under the floor, three policemen enter the house. The narrator is so confident that his crime will not be discovered that he invites them to sit down, right over the place where the corpse is buried. Although he laughs and chats with the policemen at first, soon he becomes distraught. He believes he hears the same sound he heard previously, which he thought was the old man's heart. This disturbs him greatly, and finally he shouts, "I admit the deed!—tear up the planks!—here, here!—it is the beating of his hideous heart!”
One interpretation of the story is that some supernatural process is at work. The narrator suggests near the beginning of the story, "I heard all things in the heaven and in the earth. I heard many things in hell." Thus the heart he hears at the end of the story may be a demonic sound or it may be a heavenly sound--one designed to bring judgment on him for his horrible sin.
Although that is one interpretation, it is not the probable one. The narrator is the quintessential unreliable narrator; he claims he is not mad, and the more he insists, the more the reader believes he is. If he is mad before he kills the man, then his perceptions may be warped. The sound he thinks is the man's heartbeat may be his imagination, or it may be his own heart. As he becomes more and more angry at seeing the man's "vulture eye," his heart beats more loudly so that he can "hear" it within himself, even though he is really only feeling it. As he gets calmer as the man dies, his heartbeat slows, and he can no longer hear it. But when he is sitting with the policemen, he begins to get nervous, and his heart beats more quickly. Again, he feels his own heartbeat but thinks he hears it, causing him to make his confession.
So why did Poe choose the title he did? He could have named it after the narrator's own heart, which was the "tell-tale" heart, like a tattle-tale, that gave him away. Since the narrator is the one who is telling the tale to begin with, using the term "tell-tale" is a pun that gives a clue to the story's interpretation. The title also alludes to a famous saying, "Dead men tell no tales." That is ironic in this context because in the narrator's view, the dead man did "tell" on him. The saying is also often used as a motive for murder, so using the term in the title also signifies the story is going to be about a violent murder by a ruthless killer.