Ad hominem is a logical fallacy that involves a personal attack: an argument based on the perceived failings of an adversary rather than on the merits of the case. In short, it's when your rebuttal to an opponent's position is an irrelevant attack on the opponent personally rather than the subject at hand, to discredit the position by discrediting its supporter. It translates as "against the man."
Using an ad hominem fallacy pulls the public's attention off the real issue and serves only as a distraction. In some contexts it's unethical. It's also called argumentum ad hominem, abusive ad hominem, poisoning the well, ad personam, and mudslinging. The attacks serve as red herrings to try to discredit or blunt the opponent's argument or make the public ignore it—it's not just a personal attack but one stated as a counterattack to the position.