Check this option if you want to be allowed to apply filter rules to any mailbox. Normally, filtering rules are only applied to new mail that is delivered to your Inbox. If this option is checked, an icon will appear on all mailbox pages that will allow you to apply filter rules to the messages in the current mailbox.
Filtering in mailboxes other than Inbox may produce strange (e.g. the filter may cause a copy of messages to be delivered to the same mailbox) and/or undesired results (e.g. it may cause messages to be moved/deleted if new filtering rules have been implemented since the message has been delivered to the current mailbox).