It can be because you may have moved content from one domain to another and need to redirect traffic from the old domain to the new one. You may have multiple domains that you want to redirect to a single website so for whatever the reason you wish to do it here are ways to do it.
Redirect URL using META TAG
Add this meta tag to every page you wish to redirect to another URL
<meta http-equiv="refresh" content="0;URL= http://letsforum.com">
Redirect URL using .htaccess file.
This redirect is mostly known as 301 redirect so open up notepad copy and paste this code then save it as .htaccess make sure to use ALL FILES as Save as type
RewriteRule ^(.*)$ http://letsforum.com/$1 [L,R=301]