Put simply the AA platform is an Interface that acts as an Intermediary between FIU's and FIP's through a set of standardized APIs.
In simpler terms between Financial Institutions and consumers of Information.
Financial Information User (or The FIU) ⇒ These are the users of the information, normal people, other institutes, and small businesses.
At the moment (To maintain balance in the universe) a FIU has to be registered with the RBI.
Financial Information Provider (or The FIP) ⇒ These are the banks, the Insurance Agencies, Stock Lenders, etc (i.e Data Access Fiduciary).
The Account Aggregator (or The AA) ⇒ The AA is what links the FIP and the FIU, it has the ability to get data from the FIP and then make that data available to the FIU, all in an Encrypted, Secure and granular manner.
Any authorized AA can talk to any FIP.