I don't even know what a mail forwarding file is. What is it?

If you have a shell account on a machine (and the associated mail account, say foo@bar) you can redirect mail arriving to that account to some other account (like an account where you read mail more regularly). Or just have it send a copy somewhere else, or to multiple places.
The very simplest form is just a single line with a mail address on it - that forwards all incoming mail to that address.

You can also have it process your mail through filtering/processing software like procmail.

This is all set up in a text file named ".forward" that lives in your home directory.
Another file controls how the procmail program should work, etc etc...

/Michael