Ad hominem literally means "at the person". It's the most direct way to attack someone's character or credibility, by saying that they are wrong because of who they are.
This example comes from the Suze Orman Show, where Suze gives financial advice to callers:
Caller: We have talked to a financial advisor and he recommended that we put it in an annuity.
Suze Orman: [...] Did that advisor also say to you that if you put that $80,000 in there I’m gonna make about $4,000 of commissions? Did he or she happen to tell you that as well?
(Source: Last Week Tonight - from 4:40)
Looking more broadly at persuasion—not just argument and logic—bringing up the financial advisor's conflict of interest is totally relevant, because they could be abusing their power as an expert (and thus their audience's willingness to take the things they say for granted).
But from the standpoint of pure logic, who someone is and what they do doesn't affect whether they are right. Are annuities a bad idea? More evidence and reasoning are required!