In more detail
A slug is the human-readable last part of a URL that names a page, such as how-hashing-works in a path. Good slugs are short, descriptive and stable, use hyphens rather than underscores or spaces, are lowercase and drop accents and punctuation. Search engines read the words in a slug, and people read it when deciding whether to click. Changing a slug later breaks existing links unless you add a Redirect.
Why it matters
A slug becomes part of the address people bookmark and share, so it is hard to take back. Stuffing dates or categories into it forces a change when they no longer apply, and unsafe characters get percent-encoded into unreadable %C3%A9 sequences. Two pages with the same slug in the same place collide, so generators often append a number.
Example
Turning a title into a slug normalises accents, lowercases, replaces every run of other characters with one hyphen and trims the ends:
Café Menu: 10 Best Tips! -> cafe-menu-10-best-tips
Crème brûlée — the Ultimate Guide (2026) -> creme-brulee-the-ultimate-guide-2026
The Text to Slug does this in the browser.
Common confusion
A slug is only one segment of a URL, not the whole address. The domain, the path and the query string are separate parts.