Basically the referrer check verifies when they post, they they were posting from a page on your site, and not some kind of bot or offsite spamming script. But if the user has a firewall (hardware or software) and it's blocking the sending of the referrer information, then the script can't tell it's a legitimate reqest, since there's no info. Thus it gets blocked.
For firewall users, if you leave the check on, they will have to add your site to a list of trusted sites if they want to post, so the referrer info is sent.
To make it easy on users, you can disable this. Many sites run with this disabled.
