A Bonus is a number with a "+" sign. Bonuses are added to die rolls or to statistics. Most bonuses stack with each other, but not with themselves.
For example, if a character has cover from more than one source, it still gets only a +4 bonus to Defense. A Clone Trooper Commander can give nearby trooper followers a +3 bonus to Attack, but two Clone Trooper Commanders together can't give a trooper follower a +6 bonus.