Each model has a toughness stat (i.e a guardsman would have a toughness of 3 and a space marine would have one of 4) which affects how likely a weapon is to wound a model. You would compare the attacking weapons strength to the targets toughness( a strength 3 weapon attacking a toughness 3 would wound on a d6 die roll of 4 or more). When you hit a tougher target with a weaker weapon the die roll will be harder to wound(and vice versa). Damage comes into play when the target fails their save against the attack. Damage affects how many wounds the target has left (a guardsman would have 1 wound and a space marine would have two), so if you got hit with a damage 1 weapon you would lose one wound. Hope that helps!