Then explain it.
Are you thinking of "speed limit"?
No, I said "speed of light".
Then explain it. [relativistic time dilation]
"the objective measurement of time is different depending on your time frame" is about as concise as I can get. Time measured varies with speed of different frames of observers. Distance to gravitational mass can also be a factor. I'm really kind of lost what you're looking for, but Wiki is pretty brief and hopefully informative enough here.
No, I said "speed of light".
I thought maybe you were going for the speed of light being the speed limit. If that wasn't it, Einstein said quite a bit about light; I'm not sure where to look next. Can you say what you had in mind?