Externalities are the good or bad effects that are caused by a company's activity, but they don't actually cost or benefit the company. For example, if a factory produces a lot of pollution, that pollution could cause harm to other people living near the factory, but the factory isn't directly responsible for paying for that damage. You could say the damage is an external cost of the factory's activity. On the other hand, if a company plants a lot of trees that provides shade for people in the area, that benefit is an external benefit of the company’s activity.