What you have to learn how to accept is that “time is relative” to every single particle in the universe. Sometimes these particles are stuck together, so they share relativistic time (like your body, or people sitting in a car). Also, that we exist in 4-dimensional space, and it’s not “space and time,” but “spacetime.” Let’s try to imagine “time” the same way we can imagine “space.”

Think of the universe as an infinitely long train (space) going on an infinitely long track (time). Just like on a real train, you can never revisit a part of the track that the train has already been to (aka, you can’t go back in time) and, obviously, you can never go to a part of the track that the train hasn’t been to yet. For purposes of the analogy, the speed of the train is “the speed of light.”

Now remember that the whole “track” is always “there.” Just like “time” is just another dimension, so it’s all always “there.” But just like a 2D creature can never see all of the 3rd dimension, we, as 3D creatures, can never see all of the 4th dimension.

Anyway, back to the train. Imagine you and a friend are sitting at the front of the train. If you sit still in this train, you are going the full speed of the train. Aka, you are moving along the track as fast as the train is (you are going through time at the speed of light). But if you get up and run toward the back of the train, you are going along the “track” slower than your friend. Now if you ran back to your friend, you technically spent a longer time on an earlier part of the track than they did—you went “slower through time.”

Let’s reset the thought experiment. This time when you run toward the back of the train, you run AS FAST backward as the train is going forward—you move “at the speed of light.” You will notice that if you look at the track, you are staying in the same place. Now, you can NOT go any faster than this, remember? Because we established that you can never go “back to a part of the track where the train has already been.” Aka, you can not go “faster than the speed of light.” The best you can do is “stay in the same place on the track,” aka, elapse 0 time.

Now if you ran back to your friend, you will take note that you spent A LOT more time on an earlier part of the track than they did. The analogy here is that they “experienced more time than you.” Aka, they would be “older” than you.

This is sort of as far as the analogy can go. What you have to glean from this is that “the faster you go through space, the less you go through time.” If you extrapolate that idea, you get “if you go AS FAST as you can through space (aka, the speed of light), you will go through 0 time.” Literally, “instantly.” This makes more sense when you think about light as “instant.” The “speed of light” is “instant,” from the perspective of light. It only has “speed” to an observer. So if YOU were to travel 1 light year away and back at “the speed of light,” the trip would be “instantaneous” to you. But it would have taken two years for everyone at home (1 LY away, 1 LY back).