What is the ad hominem fallacy ("against the person")?

Rejecting a claim by attacking the person who made it instead of the claim itself — the person's character has no bearing on whether the claim is true.

"Ad hominem" is Latin for "against the man." The move is: Person A makes claim X; Person B attacks A's character, motives, or circumstances; therefore X is treated as false. It fails because the truth of a claim is independent of who says it — even a repugnant or biased person can state a true claim, and even a saint can be wrong.

"You say we should raise the minimum wage, but you've never even run a business — so you're wrong."

Whether the speaker has run a business is irrelevant to whether the policy claim holds; that has to be settled by the arguments about wages, not by the speaker's résumé. Attacking the arguer dodges the actual argument.

Tip: Ad hominem is a fallacy of relevance — the attack might be perfectly true and still prove nothing about the claim.