I think it's "time traveling forwards happens slower the faster you move."
The easy explanation is that we're traveling in a 4 dimensional space at the speed of light, and any increase in speed in the 3 spatial dimensions must subtract from your speed in the orthogonal time dimension.
If I jump on a ship and travel near light speed (Not possible yet) and then land on the same spot I would not have spent the same time as those on the earth.
Now at a tiny level anyone ever on an airplane also will have spent less time than those who never have flown.
The easy explanation is that we're traveling in a 4 dimensional space at the speed of light, and any increase in speed in the 3 spatial dimensions must subtract from your speed in the orthogonal time dimension.