An ad hominem attack is a form of argument where the truth statement of a claim is based on the character of the claimant, and not on the intrinsic virtue or vice of the claim itself. That is, rather than refute the assumptions, premises, or logic of an argument the argument is cast aside because of some perceived negative attribute of the person putting forward the argument. This doesn’t mean that merely insulting a person makes something an ad hominem attack (though it may be lazy argumentative skills), but rather the insults are used as a justification for making a truth statement about a claim.
“Raising the minimum wage increase unemployment.” “Well you’re a right-wind ideologue so that isn’t true.” Rather, the claim itself should be evaluated using economic theory and compared to empirical observations, which may depend on environmental context. “How can so many people be fooled by faith healers?” “Because they’re stupid.” The trick employed by such con artists are subtle and use applied human psychology, so it’s incorrect (and a bit conceited) to write it off as low intelligence. “Bill says that perpetual motion machines can’t exist.” “He would say that, he’s a skeptic! They think everything’s impossible.” The laws of thermodynamics are arguably the most unassailable of discovered natural laws, and whether Bill is or is not a skeptic is irrelevant to the physics involved.
There’s a broader trend here with some of the previous fallacies: the need to divorce a position from the person putting forward the position in order to make a sound argument. While it’s certainly true that the different characteristics and attributes of a person can affect what claims they might make, the validity of those claims needs to be evaluated in their own right, apart from whatever motives the claim’s parent might be.
In practice, I don’t think this means that the onus is on skeptics to evaluate in great detail every claim thrown their way. There is the issue of “the boy who cried wolf,” that after a while some person might have bankrupted their intellectual integrity account and further argument or debate is meaningless. While true scientific skepticism would be totally agnostic about the origin of all claims, there’s only so much time in the day.