A trajectory is the direction in which a story is currently moving. Trajectories may be stable or they may fluctuate, depending on the play styles of the group. Understanding an RPG's trajectory is useful for GMs to prep upcoming sessions.
In role-playing games, there are two main approaches to using a trajectory. The first approach is to develop a plot, including encounters or scenes that the players will come across, and to tweak the details of these as the trajectory moves. The other is to play to find out what happens, for which prep focuses more on developing a setting that can react to player choices.