A redirect is an automatic forward: when someone opens one web address, they are instantly and invisibly sent to a different one. A “301” redirect is the permanent kind, used when a page has moved to a new address for good.
It works just like the mail-forwarding service you set up when you move house: letters sent to your old address are quietly passed on to the new one, so nothing is lost and no one has to know the change. On a website, this means old links and search results still lead people to the right page instead of a dead end.
Redirects are important whenever a Domain or a page address changes — for example, after a redesign or a move — because they keep old bookmarks working and let search engines carry a page’s reputation over to its new address.