A slug is the readable part of a URL that identifies a page - the 10-essential-seo-meta-tags in example.com/blog/10-essential-seo-meta-tags. Getting it right at publication time matters more than most single decisions about a page, because changing it later means either breaking every existing link or maintaining a redirect indefinitely.
What makes a good slug
Lowercase. URLs are case-sensitive after the domain on most servers, so /My-Page and /my-page are two different addresses. Publishing mixed-case slugs invites duplicate content and broken links from anyone who types the address by hand.
Hyphens rather than underscores. Google has been explicit about this: hyphens are treated as word separators and underscores are not. seo_meta_tags reads as one long token; seo-meta-tags reads as three words.
Short, but not cryptic. Long URLs get truncated in search results and are awkward to share. Aim for three to six meaningful words. The default limit here is 75 characters, and when a slug has to be cut it is trimmed at a word boundary rather than mid-word.
Stable. Decide the slug when you publish and leave it alone. Editing a headline afterwards does not require editing the URL.
Accents and non-Latin text
Accented characters are transliterated to their closest ASCII equivalent - café becomes cafe, Müller becomes muller. Technically modern browsers handle Unicode URLs, but they are percent-encoded when copied, which turns a readable address into a wall of %C3%A9 sequences the moment anyone pastes it into a chat message or an email.
Text in a script with no Latin equivalent - Arabic, Chinese, Cyrillic - has no sensible transliteration and will be stripped. For those languages, write the slug in your own words rather than deriving it from the title.
Stop-word removal
The optional stop-word filter removes common short words: a, the, and, of, in, on, to, for, is and similar. The Complete Guide to the Best Free Online Tools becomes complete-guide-best-free-online-tools.
Use it with judgement. Shorter slugs are generally better, but stop words sometimes carry meaning - the-office is not office, and removing not from a title inverts it. The filter keeps at least one word if removal would empty the slug entirely, but it cannot tell when a small word was load-bearing.
Bulk mode
Turn on bulk conversion and each line is processed separately. Paste a column of post titles from a spreadsheet and get a column of slugs back, which is the practical way to prepare a content plan or migrate a set of pages without doing them one at a time.