Attacking the person making an argument rather than the argument itself. By discrediting the speaker, the attack implies (without demonstrating) that their claims are wrong.
In a policy debate, instead of engaging with a scientist's climate data, a commentator notes that the scientist once accepted funding from an environmental advocacy group. The implication: the data is compromised. This is possible — but the data needs to be evaluated independently, and dismissing it by attacking the source is not an evaluation.
Source credibility is a legitimate (and necessary) input to how much evidence we need before accepting a claim. But ad hominem short-circuits this by treating the person's character as a replacement for evidence rather than a factor in weighing it. Once someone is labeled biased, corrupt, or stupid, audiences stop processing what they actually said.
Watch for responses that lead with personal attacks, credentials-questioning, or character accusations rather than engaging with the substance of a claim.