1) :: represents Alternate states, Group1 alternate state is equating with default state Region values.
2) If you want to group by dimensions then you can specify like this total < Month, Group >, groups the sales value by Month and Group.
3) Here # represents number, it is not mandatory.
I don't want to sound dismissive, but you can find all of that in Reference Guide, and I honestly doubt my own explanation would be better in any way ;-) Specifically:
1) page 844, Alternate States in Set Expressions
2) page 782, Chart Aggregation Functions (examples on page 843)
3) page 281, Dollar-Sign Expansion Using a Variable
(page numbers from Reference Guide for QlikView 11 SR2, 11414)