301 Redirect
A 301 redirect is a permanent, server-side redirect that automatically sends visitors and search engines from an old URL to a new one; Google treats it as a signal that the new address should be canonical and shows the redirect target, rather than the old URL, in search results.
In practice
A 301 redirect is indispensable as soon as a URL changes, whether through renamed pages, a website restructure or a domain migration. Without one, visitors and search engines land on a dead link at the old address.