Difference between master site and materialized view site in oracle
Let's say I have two databases, DB1 and DB2. I want to replicate data from DB1 to DB2. I created materialized views on DB2 and these materialized views gets data (periodically) from tables stored on DB1. 

So which is master site here?
The database from which replicated sites(DB2 which has materialized views) gets data. In other words the database which stores master tables , which is DB1 in our case.

then materialized view site?
The database which stores the materialized views, which is DB2.

Documentation:
About Master Sites, Master Tables, and Materialized View Sites