Knox is a technology that Samsung has in place to detect tampering. For security-conscious environments, like government or corporations, it allows them to better practice good security policy. If Knox is installed, it will detect if the recovery partition has been changed. You can still upgrade and root 4.4 and not trip Knox, but you would not be able to install alternate ROMs. Doing so would trip Knox, and may affect your ability to get warranty work done. Since 4.1 doesn't have Knox, the poster you replied to suggested that he would keep the bootloader version at 4.1, but upgrade the system partition to 4.4. Hence, you would have the latest Kit Kat installed, but without the Knox bootloader.
That said, I've only had an Android for a few days now. Also, you can get brand new Virgin Mobile S3's with the 4.3 firmware on eBay for $170. If I had known about QVC I would have gone that route and had a Knox-free phone. Ultimately, though, It's really not that big of a deal.