When an argument is met not with revealing factual errors, revealing logical errors or offering workable alternate explanations, but instead with an attack against the person, this is an ad hominem (lit “to the person”).
Our Australian political debate was full of ad hominem fallacies. Instead of addressing the topic, the politician attacked their opponent.
A common example is this:
Argument: “Climate change is no longer a scientific question, it is a scientific fact due to the overwhelming evidence that supports it”.
Ad hominem fallacy: “You must be stupid to believe such trash”.
The intelligence of the person has nothing to do with the validity the argument. Instead of attacking the argument, the respondent is attacking some aspect of the person. It makes just as much sense as saying “your point is invalid because you have blue eyes”.