Amazon RDS Read Replication enhances the performance and durability of database (DB) instances. This replication capability makes it easy to elastically scale out beyond the capacity limits of a single DB instance for read-intensive database workloads
Sharding is a general concept for dividing data among multiple tables in a database. When the operation of a single master DB is limited due to an increase in requests, etc., access can be distributed by allocating data to multiple DBs according to certain rules.
For more information on sharding in RDS, please refer to the following