It’s simple: You are familiar with how temperature works, right? Imagine the same volumetric difference exists in 3d space, but the different regions are not h

It’s simple: You are familiar with how temperature works, right? Imagine the same volumetric difference exists in 3d space, but the different regions are not hotter/colder, but time goes faster/slower. So You get two identical clocks, showing the same time, and move one of them into a “slow time” region (which would be a star moving very fast (8% of lightspeed). Let some time pass, then re-join the two clocks. Since they spent some time in regions with different “speed of time” they’ll show a different time. I might be wrong though, I’m not a professional.