Sometimes, we might think that we can just skip over appendices—it's like the book is the main feature and the appendices are extras. In the Enquiry, though, the appendices are super important. As Hume rounds up the final one, he focuses on language. Language is something that we can't get away from: we're all born into it and become familiar with popular lingo. However, this doesn't mean that it's perfect.
At the end of this appendix, Hume mentions some of the words we may use to describe faults or bad behavior, like "a blemish," "a vice," and "a crime." These terms may signal different levels of error but, for Hume, they're part of the same species.
Hume's concern is that philosophy can sometimes get bogged down in terminology and that this can lead to confusion. Hume argues that explaining one of the above words leads us to an understanding of the others and that, ultimately, there are more important things to focus on than words.
So, what is important? Well, for the Enquiry, it's morality, regardless of what terms we use to describe it. Even the most basic systems of morals recognize that we have a duty to ourselves, and this leads us to explore the relationship between this duty and our duty to society.
Rather than leaving this topic hanging, Hume reaches a pretty solid conclusion: both duties probably develop from similar principles and attract similar praise. The ending therefore does a great job of wrapping up the Enquiry and summarizing Hume's take on morality. Ignore it at your peril!