What is a 301 redirect
In short
A 301 redirect is a permanent redirect from an old URL to a new one. It tells both the visitor and Google that the page has moved permanently to a new address.
When a page moves or a site is rebuilt, URLs change. Without a redirect the visitor would hit an error page and the old page's SEO value would be lost.
A 301 redirect smoothly sends the visitor to the new page and passes most of the old page's SEO value along.
Why a 301 redirect matters
When updating a site, the 301 redirect is what preserves SEO rankings. Without it a new site would lose the old site's accumulated value and start from zero in search.