You will need to be more specific than that since the BI stack is pretty much the most diverse and complex one that Oracle has.
Do you mean architecture? Servers?
I would like to know about the architecture how BI works
....as a database administrator what would be my role?
Your role? Sit down and enjoy your coffee
As a DBA from a BI point of view you don't really have an active role as long as your BI developers know their job. In a more realistic situation you will probably have to help diagnose slow queries and find out if by adding indexes or partitioning could help. Just keep in mind a detail: OBIEE doesn't execute queries written by somebody, the tool generate the queries based on a model. So as a DBA do not reply something like "add this hint or change your query to be like that" as often it's quite difficult if not impossible to force the tool in generating a query exactly as you want it (still possible but not easy).
Hahahaha not toally wrong :-)
Hadn't properly read that part so Gianni's point is extremely valid: If you have good OBI people then you yourself - as a DBA - you won't have much to do at all