What is a 301 redirect?
A 301 redirect is used to direct one URL to another. This is a way to send visitors and search engines to a different URL from the one they originally typed.
Think about it as if you're moving from one house to another house but not everyone knows your new address so you have your mail redirected via a "change of address" form at the post office.
When are 301 redirects necessary?
When you are re-platforming or re-designing your website, often times the URLs will not be the same as the last site. You will want to implement 301 redirects from all your old sites URL paths to the most relevant URL path on the new site. This should be done at launch to tell both users and search engines that your original page is no longer there and the most current and relevant page can be found on your new page.