@ pyg
If I can help you - to start, possibly others will provide more precise answers
1) It means that site searching will provide you more sites - thus more gems - thus more possibilities of forging items, summoning creatures, casting global enchantments and rituals, more mages casting big spells in combat. You're up for a more magic oriented - maybe more "spectacular"
- game of the ones you're used to, I'd say.
2) CBM makes a lot of little tweaks! It makes some spells which were lately not much efficient, more useful, and the same for some items, units, globals, and even pretender gods. Some overpowered units are made a bit less cost-efficient, though (i.e. the Cyclops pretender, which was not only a mighty SC but even powerful for double blesses and magic, now he's still strong and a Earth powerhouse but not much more) . The mod comes with a very useful readme, read it to understand how it works!
The scales are tweaked, notably the heroes are better so the Luck scales is more useful, but I remember there were other tweaks too but they really don't come to my mind now
Have fun!