The special theory of relativity predicts that events are simultaneous for observers in one inertial frame of reference are not necessarily simultaneous for observers in a different inertial frame of reference.

The special theory of relativity predicts that if you are observing events in an inertial frame of reference that is moving rapidly relative to you, times in that frame of reference will appear to slow down.

Relative time - Time measured in frame moving relative to actual event.

Proper time - Time measured in the same frame as the event itself.