Putting the joint through the full range of each motion allows for maximal stretching.
--- Can anyone try to write it/rephrase it in a simple way for me?
I can try to explain it if that's what you want.
A joint is a part of your body that bends, like your elbow or your knee. Putting it through the full range of motion means twisting it around every which way it will go. If you move your joint around as much it will go, you are then you will be doing the "maximum stretching" or the most stretching possible.