What does the title "The Tell-Tale Heart" mean?

At the most obvious level, the title refers to the beating of the old man's heart. The heart "tells tales" to the narrator. Tales, as you well know, are stories, and can be based on either real or imagined events. In either case, tellers of tales want to keep the reader or listener paying attention, and will often resort to extreme exaggerations to achieve that goal.

So, what tales does the old man's heart tell? We first hear his heart beating on the eighth night, when he realizes that something is not right in his room. His heart tells a tale of fear, which in turn makes the narrator extremely angry and gives him the push he needs to carry out his dastardly deed. 

The next time we hear the beating of the heart is after the old man is dead. See, this is part of why the narrator tells us he cut up the body before burying it under the floorboards. If it wasn't for that step, we could imagine that the old man maybe wasn't quite as dead as the narrator thought. Since that isn't a possibility, and since we know that dead hearts don't beat, the narrator's own hidden guilt over the deed is projected onto the dead man's heart, thus telling a tale of the narrator's guilty feelings.

So, the title also refers to the narrator's heart. Inside the heart is where our deepest, truest feelings and emotions live, at least metaphorically speaking. We could look at the whole story of the old man's murder as a tale told by the narrator, a tale from his own heart. The title refers to both the narrator's heart, and to the old man's heart, and to the tales told by both.