Plot means the drawing of a series of events in a story. In a novel, the plot is decided by the author. In an RPG, however, the plot is a prediction of future events and may change, even radically.
In a linear game, the GM makes provisions to keep the plot on course. In a sandbox gamme, the GM prepares characters and information that allow the characters to have an adventure whatever they decide to do, even if it means the game takes a surprising turn.