The usual way to filter your email is to simply check email in your email program. Your email program will connect to Spamfire, Spamfire will retrieve mail from your email server, and Spamfire will deliver the good email to your email program.
One problem with this approach is that if Spamfire fails to identify any spam messages, those messages will be delivered to your email program. Of course, you can (and should) open Spamfire and correct these mistakes. But the messages will still be in your email program. Some people find this aesthetically displeasing.
Instead, you may sometimes find it useful to pre-filter your email. Instead of opening your email program, open Spamfire first and choose Start Filtering from the View menu. When Spamfire is done filtering, you can choose Show Good Mail from the View menu to see all the messages that Spamfire thought was good. If any of those messages are spam, you can correct them now.
After you have corrected any mistakes, open your email program. It will download only the good mail and will respect any corrections you made.
By pre-filtering in this way, you can always prevent spam from reaching your email program.