301 is an HTTP status code sent by a web server to a browser. A 301 signals a permanent redirect from one URL to another, meaning all users that request an old URL will be automatically sent to a new URL. A 301 redirect passes all ranking power from the old URL to the new URL, and is most commonly used when a page has been permanently moved or removed from a website.
Google Officially on 301 Redirects
When should you use a 301 redirect? Google recommends using 301 redirects in the following instances:
Changing a domain or domain name. If users know your brand and direct traffic is high enough, you will avoid confusion with a new address.
You are updating an old site or migrating to a new site. When users go to outdated pages, they must be redirected to the newest version.
A 301 redirect makes your old links go to the new location. This means the old page is removed from Google's index and most of its link equity transfers to the new one.