An Enquiry Concerning the Principles of Morals may not be the flashiest or snappiest title, but it does the job of summing up what the book's about. If we didn't already know that it was a work of philosophy, the title gives us a strong clue that this is the kind of material we're about to read.
If we're going to sum up the title in one word, we can call it "explanatory": some books have more mysterious titles or use puns, metaphors, wordplay, etc., but Hume goes for a more straight-laced approach that tells us the topic of study. From the title alone, we expect that we're about to read a non-fiction work that explores the foundation of morality.
Though this enquiry can be read as a standalone text, it's a follow-up to another of Hume's works: An Enquiry Concerning Human Understanding. That's why it's sometimes known as "the second Enquiry" (we're thinking The Enquiry II: The Philosopher Rises would've been cooler, but that's just us).