The principle which cause has to always preceed effect. More formally, if an event A ("the cause") somehow effect an event B ("the effect") that occurs later in time, then in turn event B cannot contain an influence over event A. i.e., event B has to occur at a later on time t than event A, and further, all of frames has to agree upon this ordering.
The principle is best illustrated through an example. Say that event A constitutes into a murderer taking a decision to kill his victim, and that in actually event B is the murderer committing the act. The principle of causality puts that the act of murder cannot contain an influence over the murderer's decision to commit it. If the murderer were to in some way see himself committing the act and change his mind, then murder would have been committed in the future without a prior cause (he changed his mind). It represents a causality violation. Both time travel & faster-than-light travel both imply violations of causality, which is why most of the physicists think they are not possible, or at least impossible in the general sense.