Someone who understand what TUT and progressive overload is?

From what I know so far, I have to pay attention to those things when trying to build muscles. But there is something I don't get. I am suppose to lift weights under my max in in 6-8 reps in several sets and it has to be in a certain time limit, but from what I know about progressive overload you are suppose to lift as much as you can and keep increasing. So what am I suppose to do?