Two people in relative motion have their own distinctive times. According to mission controllers on Earth, an astronaut in a spacecraft travelling to a distant planet will have a time that runs more slowly than their own. This is called time dilation. Lengths are also affected by speed. According to the astronaut, the distance between the earth and the planet is less than it is for the mission controllers. This is length contraction.
In spacetime, all of time (past, present, and future) exists on an equal footing - just as all of space exists on an equal footing. This would imply that the future, in a sense, already exists; it is just waiting for us to come across it consciously. But is spacetime truly real? Is it the reality? Might it not be just a convenient mathematical device? Opinion among physicists remains divided.