An e-mail forwarder is a an e-mail mailbox which automatically sends incoming messages to one or more external e-mail addresses.
For example, if your name is John Smith and your domain name is smith.com, you might setup an e-mail account (mailbox) called jsmith. The mailbox will receive e-mail sent to jsmith@smith.com. However, you may want your partner to also receive all messages sent to jsmith@smith.com. Assuming your partner's e-mail address is jdoe@smith.com, you can configure your jsmith mailbox to automatically send a copy of each message also to jdoe@smith.com. You can also forward messages to external e-mail accounts such as jsmith@yahoo.com or jsmith@hotmail.com.