Time is not absolute but is relative to the motion between the observer and the event being observed.
Time dilation is the apparent slowing down of time of a body moving at relativistic speeds.
The essence of the time dilation phenomenon is that a lapse of time as recorded on a single clock is compared to the results of measurements in a reference frame relative to which this clock is moving. In any such comparison, it emerges that the lapsed time as measured on the single clock is less than the difference between the two clock readings that describe the beginning and end of this interval in the other frame. The time-dilation phenomenon loses most of its mystery once we recognise that it is basically the consequence of comparing successive readings on a given clock with readings on two different clocks.