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.
What Google does with it
Follows it, and uses it as a signal that the target should be the canonical page. Permanent redirects lead to the new address appearing in search results; temporary ones keep the old address in results. Google's guidance is to use permanent redirects when you are sure the move will not be reversed.
On the question everyone asks: Google's staff said in 2016 that redirects no longer lose ranking signal in the way they once did. Redirect a page to a genuinely equivalent page and it carries across. Redirect it to your homepage or to something unrelated and Google may treat it as a soft not-found instead, and the signals go nowhere.
When to use one
Changing a URL slug. Moving from http to https or between www and non-www. Merging two pages into one. Retiring a product and pointing to its replacement. Moving domains. In each case, old address to the single most equivalent new address, one hop.
When not to
For a page that will come back, use a 302 or 307 so Google keeps the original. For a page that is simply gone with no equivalent, return a 404 or 410 and let it drop out. Redirecting everything to the homepage is the most common mistake in site migrations and it is worse than a clean not-found, because it hides the loss.
Keeping them
Leave redirects in place for at least a year, and ideally for as long as the old address is linked from anywhere. Avoid chains, where A goes to B goes to C, by updating A to point at C directly. And update your own internal links to the new address rather than relying on the redirect, so the site is not forwarding itself.
Where this shows up in the tracker
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Related terms
URL slug
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.
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.
Duplicate content
Duplicate content is substantially the same content available at more than one URL, on your own site or across sites. There is no duplicate content penalty. What happens instead is that Google picks one version to index and folds the others into it, and if it picks the wrong one, or splits links between them, you lose some of what you have earned.
Sitemap
An XML sitemap is a file listing the URLs on your site that you want search engines to find, usually at yourbrand.com/sitemap.xml. Google says it helps discovery and does not guarantee that anything in it will be crawled or indexed. A small, well-linked site may not need one; most sites should have one anyway, because it is free and it is how you tell Google what changed.