The URL slug is the part of a web address after the last slash that identifies the page, such as wedding-fireworks in yourbrand.com/wedding-fireworks. Google recommends readable words separated by hyphens. It also says the keywords in a URL on their own have hardly any ranking effect, so the slug is for clarity, not for gaming.
What Google recommends
Readable words rather than ID numbers, in the language of your audience, separated by hyphens rather than underscores, and no longer than needed. That is the whole of the official guidance, and most sites break at least one of those rules on every page a content system generates automatically.
On ranking, Google's SEO starter guide is unusually direct: the keywords in the domain or the URL path alone have hardly any effect beyond appearing in breadcrumbs. Google's staff have described it as a very small factor for years. The slug helps people and helps you keep your own site organised. It does not carry the page.
Why it still deserves five seconds of thought
Because it is visible. The slug appears in the result, in the browser bar, in shared links and in anchor text when someone pastes a bare URL. A slug that says what the page is earns more clicks than one that says post-4471, and it makes the link more likely to be reused.
And because it is a commitment. Once a page is indexed and linked, its address is part of how Google identifies it. Changing the slug creates a new URL that has to be redirected from the old one and re-evaluated from scratch, carrying its signals across through the redirect but starting again in the queue.
When changing a slug is worth it
Rarely, and deliberately. If a page was aimed at a phrase far too competitive for the site and you are repositioning it at a narrower one, changing the slug along with the title and heading makes the repositioning coherent. Redirect the old address permanently and expect a few weeks of turbulence.
Do not change slugs to freshen pages, to add a year, or because a plugin suggested it. Each change is a new URL, and a site that keeps renaming its pages keeps resetting them.
Conventions worth adopting
Lowercase throughout. Hyphens between words. No stop words unless the phrase reads badly without them. No dates in evergreen content. Folders only where they reflect a real structure a reader would recognise. And decide once, for the whole site, whether URLs end in a slash, then never vary it.
Sources
Where this shows up in the tracker
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Related terms
Title tag
The page title is the text in a page's title element, shown as the clickable headline in search results. Google uses it as the main source for that headline, along with the visible heading, link text and other prominent text, and will rewrite it when it judges the title unhelpful. A good title says specifically what the page is, in the words the searcher used, without padding.
301 redirect
A 301 redirect is a permanent instruction that a page has moved: anyone requesting the old address, including search engines, is sent to the new one. Google treats it as a signal that the new address should be the canonical version and shows the new address in results. Use it whenever a URL changes for good, and keep it in place.
Canonical
A canonical tag is a line in a page telling search engines which address is the official version, for when the same content is reachable at several different URLs.
Internal linking
Internal linking is linking from one page on your website to another. It is how Google discovers most of your pages, how it works out which ones you consider important, and how it learns what each page is about from the words used to link to it. It is also the one ranking lever that costs nothing and that you control completely.