What it means is that, if you have 18 str or less and 0 IF (6 is minimum threshold) and wear no armour, you don't get wpf penalty, and, if you use armour, but it doesn't weigh more than 6 (minimum) (where head armour weight counts x2, body x1, hand x4, legs x1), then you don't get penalty either. If you wear armour that is heavier than 6, then you get wpf penalty equal to the difference between 6 and the rest of the weight.
If you have 24 str and 5 IF, then your weight cap would be 13 (24/3 + 5).
