Call Forward is one of the standard three "terminator" actions that sends a call out of the current call path directly to another phone number of your choice. The most common use for this option is to forward a call from a routing action (last caller, record owner, or rep queue) where no one picked up the call. In this case you might forward the call to a group ring line, or a number where the caller can leave a message.
Depending on where you forward the call, you might want the recipient to see a specific caller ID on the receiving end. You have three options: the original caller's number, the phone number established as your company's default caller ID in your IS.com company profile, or a manual number of your choice.
Obviously you'll want to choose the number that will give the recipient the best idea of where the call came from and how to best handle it.