A “slug” is how a page title is encoded as a URL. Unfortunately, many special characters, include spaces, aren’t valid in a URL and need to be encoded. Spaces get encoded as %20, which is hard to read. If I had used this page’s title as its slug, it would be encoded as slugs%20make%20links%20readable. Gross!
Instead, I define a custom slug for my published notes that doesn’t use characters requiring encoding. No apostrophes, accents, or spaces for me! This page for example has the slug slugs-make-links-readable. It’s not perfect, but it’s a lot easier on the eyes than the version gunked up with a bunch of %20s in it!
This is why sites and publishers online will typically use a hyphen or underscore in the slug; it’s more readable! That readability isn’t just for humans, either. Search engines prefer content that they think will be easier for a human to read too.