A 301 redirect is a response status code informing search engines and browsers that a web page or even an entire website has been permanently moved to a new location (URL). A 301 will automatically redirect the user to the new URL and is the best way to insure that search engines are directed to the correct page.
A 301 redirect is extremely important if you have an existing site with traffic that you want to send to another website. 301s will also help you maintain your existing search engine rankings so you do not lose traffic after the transition to the another URL. Examples of why you would need a 301 redirect include: