Its about choosing words carefully; being tactful would mean not making it so obvious of the intent of the message. If I were to call you a idiot, that wouldn't be very tactful. If I were to say that you could do better academically in the subjects of science, reading, and math, however, it becomes harder to realize that its something of a insult, which is being tactful.
Not that you actually need any help with that stuff; I'd be the first person to say about your good reading skills, and your math isn't too bad. Cant say about science, though...
And yeah, it CAN have to do with being polite, but not always.