Fairly simple really: It is an instruction in a website that sends all traffic from the called URL to a new URL. A called URL is a web page address that is entered into a user’s browser or exists as a link and is requested from the domain. In plain English: When you try to visit a web page that either no longer exists, or has permanently moved to a new address, this instruction by the website tells your browser where to find the new page that replaced it or another page to go to en lieu of the old page. The new page may or may not be what you were looking for. 301 redirects are normally set when a website administrator wants to remove or relocate a page, but they are also necessary when a website structure is replaced with a new one that doesn’t (for whatever reason) allow the administrator of the website to simply re-use the same page URLs.
When setting a 301 redirect, approximately 90% of the web page’s rank is preserved until such time as the new page and its content gains (or loses) rank to a new position in search engines.
301 redirects are needed when search engines’ still hold old web page addresses that are no longer valid. They tell search engines where to find the replacement page in exactly the same way that they redirect a human visit from search.
301 redirects are also needed when pages may be called from existing bookmarked links or other referring sources that are no longer connected to the original page.
Wildcard 301 redirects can be used when very large website structures still have many URLs in search engines, but those URLs have little or no value in pagerank. This ensures that all attempts by the search engines or visitors to reach the old pages will still land on a valid page, section or category where applicable.
Wherever possible, ALL pages in any old website structure that have permanently moved should have a 301 redirect to some other valid page.
Attempts by any crawler, bot or human visitor to access a page that no longer exists and is NOT redirected will result in a 404 Error (page not found).