Passive aggressive is exhibiting anger in a passive indirect way. For example, if a customer upsets a cashier, and that cashier chooses to express her anger (and take revenge) in a passive indirect way by ringing the items up extra slowly, that would be passive aggressive behavior.
It may not always be possible to address it, if the aggressor is either a sufficient distance away (as in another car on the road) or unwilling to engage. The “victim can turn it into a direct conversation by pointing out that the other person seems upset (with the hope of remedying the hurt/disagreement), but sometimes the alternative of creating distance with space and time is the best option if the aggressor will not listen.