 Intuitively, the double cone structure at an event demarcates the speed of light. Trajectories through spacetime which thread the inside of the future lobes of these “light cones” are possible routes in which travel stays below the speed of light. Such a trajectory is a worldline and, in principle, can be traversed by a person. Now, some events cannot be connected by a worldline. But if two events  can  be connected by a worldline, there is an infinite number of worldlines which connect them. 