It allows you to spin up a relational database and have Amazon take care of the upgrades/patches/backups/etc. (database-as-a-service)
RDS instances are approximately double the price of an equivalent EC2 instance where you could self-host your RDBMS, but you'll most likely save money on developer time around managing the database
Supported databases are:
MySQL
Postgres
MariaDB
Oracle (more expensive due to licensing fees)
SQL Server (more expensive due to licensing fees)
Amazon Aurora (fully compatible with Postgres and MySQL)
