A Plot is, in essence, the storyline that unfolds as the player progresses through a game.
For example, an elderly man may be searching for a childhood sweetheart he knows is around. Yet, he can't find her. He meets the player, who he requests to deliver the message to the sweetheart. From there, the story could develop into something more or less positive, in terms of the gentleman's relationship with this sweetheart. This will be the decision, of course, of the designer of the game.