It's one of those things that all humans inherently know.
Like the color of your intestines. You're born knowing that.
Naw, I'm kidding, we're all Tabula Rasa from the start.
I suppose you may have just heard it somewhere? All I know is that you can't have a 50k polycount on some model if you plan to run a game without crashing it every 2.5 seconds.
~I have no idea what I'm doing~