The 'Define Rules' screen lets you set up rules to control what happens when email is delivered to your account. There are several sections to the screen:
Rejecting emails - allows you to reject emails sent to you based on some criteria.
Filing emails - allows you to file emails sent to you into different folders based on some criteria.
Vacation notification - respond to emails with a 'vacation response'.
Forwarding - allows you to forward emails to another email address based on some criteria.
The rules from all sections are combined together when processing incoming emails.
NOTE: When you make any changes on the 'Define Rules' screen, you must make sure you click the 'Apply all changes' button at the top of the screen. Clicking 'Back' in your browser or clicking to go to a different page will not work. The 'Apply all changes' button forces a rebuild of the internal rules script, which is what FastMail actually uses. See Advanced Rules for more details.