Connect to external device via bluetooth. Who's master/slave?
I think master means server or host but I'm not 100% sure.


It's not correct. For android device, master means client and slave means server. 
  Slave device should always [Inquiry Scan] to wait for [Inquiry] which send by master. When slave get [Inquiry], it will send FHS packet to master. And then slave begin to [Page Scan], and master begin to [Page] to slave to complete the connection.