Ad Hominem - attempting to discredit an argument by criticizing the person making it - is bullshit. But there are some things that people often call out for being Ad Hominem which are actually not. Here are two.
Flaming. Yes, flaming is (usually) bad. But it doesn't automatically constitute Ad Hominem. Ad Hominem is when you use personal insults in place of an argument; insulting your opponent alongside answering their argument, or otherwise without pretending your insults are a substitute for argument (for example as parting words when you withdraw from a conversation) - is not an Ad Hominem.
Using personal attacks to discredit something that relies on the opponent's personal character. If someone is asking you to believe them about something rather than presenting an argument that's supposed to prove it, pointing out that they've lied in the recent past is totally valid and not an Ad Hominem.