When a web server tells the client browser to obtain a certain requested page from a different location. This is controlled by directives in the server's configuration files or a "Location: header output by a CGI script.
The web server stores all its documents in a directory tree rooted at some configured directory, known as its "document root". Normally the URI part of the URL (the part after the hostname) is used as a relative path from the document root to the desired file or directory. A redirect directive allows the server administrator to specify exceptions to this general mapping from URL to file name by telling the browser "try this URL instead". The new URL may be on the same server or a different one and may itself be subject to redirection.
The user is normally unaware of this process except that it may introduce extra delay while the browser sends the new request and the browser will usually display the new URL rather than the one the user originally requested.