Download rules are applied to messages downloaded using a POP3 mail collector.
To configure the rules, go to the VPOP3 settings, then Mail Connectors → Mail Collectors → POP3 General. There is an Edit Download Rules button which brings up the download rule editor. To tell this mail collector to use the download rules, ensure that the Use Download Rules box is checked.
Download rules apply before the message is downloaded, so can only see the message headers (subject, recipient, sender, etc) and size not details of the message contents or attachments.
If you need to filter messages based on message contents, you should probably use the spam filter/content filter scripts instead
See the Reference section on Download Rules for more information.