Can anyone explain to me "render tween"?

I read the castle demo i copy the render tween way, and it's work for me.. But i want understand what it really do, i just copy it..

I rode the renderworld documentation too but i still don't understand, can anyone explain to me?
Thanks
Aza

http://www.blitzbasic.com/codearcs/codearcs.php?code=1497

Thanks puki!

Aza

Cheers, Puki.