A TAG could be described as interactive fiction (e.g. a novel or short story) that allows the user to place themselves (as the protagonist=hero) into a fictional situation and to see what the hero would see and to direct the hero's actions.
The hero is not assumed to be very bright and so the game player=user has to work out the details of how to get the hero through the situations to reach some conclusion.
What this means is better seen by actually playing a simple sample game