Is not a second a unit of measurement and constant, thus one second is equivalent anywhere? This is the concept that I don’t understand.

I’ve always struggled to get this concept and I find your explanation very good. I think what still gets me is that if a second is a second, I still don’t get why someone moving at .5c appears to have lived “300 years” when the same amount of actual time has elapsed, “80 years.” I am dense though so this isn’t a lack of teaching rather learning.