Thanks, mula is always good. And welcome Vexx007.
Stacking, means that you can have more than one piece giving the benefit and they will add up. For example, if you have two armor pieces giving +2 energy each, that would then give you +4 energy.
Non-stacking means that they do not add. For example, if you apply a Rune of Superior Domination magic to two pieces of armor, the second one will be ignored and you will still get only +3 to domination magic.
The strategy I normally follows, is to buy the "helmet" that gives +1 in the attribute I want to concentrate on. Then I add a superior rune to that helmet, so that the helmet gives +4 in effect. I then only apply minor runes to other armor pieces for the other attributes, to keep the health penalty as low as possible and a vigor rune to give some health back. That way, if I want to focus on another attribute, I just switch helmets.