Depending on the context of the redirect, it could mean many different things. If you're speaking of DNS, then a Wildcard is basically an A record (*) pointing to an IP address. A redirect would then be known as a CNAME, or alias. This would allow for any subdomain to be resolved to the IP specified in the zonefile for the * A record. If you're speaking of Apache. Then a redirect is just a simple redirect from a file to another file or url. A wildcard would match any characters of the RewriteCond you choose.