A domain redirect is an instance where a visitor is redirected to a different page when they try to access original page. This can be useful if a page has permanently moved and it is known that a large number of users may have bookmarked that page.
An example of a redirect would be if you have a file named page1.php and move that file to page2.php. You could create a redirect such that all visitors trying to access the page1.php page would be automatically redirected to the page2.php page.