I've been coming across this term too much recently. But I'm not sure what it is. I know about Alpha testing and Beta testing. I know the version of the application made available to users for and during beta testing is called a Beta release. But what exactly is Pre-beta? If they are releasing it to public for testing, it's already Beta, right?
BETA releases generally refer to full functionality and minor-no bugs expected. Pre-BETA releases are generally considered not as stable as their Beta counterparts. Some features may not be implemented and it would not be surprising to the developers to experience a Blocker issue and/or major bugs.
BETA = minor tweaking needed Pre-BETA = major development still needed
Pre-beta sounds like a non-standard term that people are inventing.
The common term for pre-beta is an alpha release, hence beta being the greek alphabet letter that comes after alpha.
Microsoft use the term "Technology Preview" that essentially means not of beta quality and features may change but hey, have a look and tell us what you think.