The point is quite clear. It's not an appeal to authority. Experts are generally more qualified to express opinions on their topic of expertise, but this doesn't make them immune from criticism. It does, however, make it a bit harder to call their view into question, and calls for a more sophisticated approach, usually by referring to problems with their evidence or absence of consensus.

Suppose an expert in a field has offered an opinion on their topic of expertise. You criticise that view based on unreliable evidence. I point out to you that your evidence is unreliable, and that the expert's view should be preferred due to its higher reliability. It's no answer for you to invoke the appeal to authority fallacy. You need to show that your evidence really is reliable, or why it's wrong to characterise your evidence as unreliable.

Suppose on the other hand that an expert in a particular field offers a view outside of mainstream consensus. I say that his view should be accepted. You say his view is not mainstream consensus. I say that he's an expert so he must be right. This is an appeal to authority.