Redirect (301/302)
A redirect automatically sends a visitor or search engine from one URL to another.
A 301 redirect is permanent: it passes on nearly all the SEO value of the old page to the new one. A 302 is temporary and does not pass on that value the same way, it tells the engine the old URL remains the reference.
Redirects are essential during a redesign or a URL change, to avoid losing the ranking already earned and to avoid 404 error pages for visitors arriving through an old link.