I'll use some silly examples.

Ad hominem - taking a claim as automatically false because of who said it:

• [Alice] "The Sun is a star, like any other."
• [Bob] "People, disregard what Alice said. Alice is no astronomer, so of course the Sun is not a star."

Ad autoritatem - taking a claim as automatically true because of who said it:

• "See that scientist there? He has a PhD, and he claims that anthropogenic climate change is not a big deal. Thus we can safely disregard it as people making shit up."

Sometimes authorities are wrong. The likelihood of being wrong might be smaller than the one of a random nobody, but it's still there. You can't simply deal with it as "authority said so then it's true". (Check what I said about inductive logic in the other comment.)

There's more, but they all boil down to "you aren't analysing the claim, you're analysing where the claim is from".