SimpleAndSuccinct is a series of succinct explanations
Amazon RDS is a service, an RDS standing for Relational Database Service offered by Amazon Web Services; AWS.
Context: Relational Databases
A lot has changed since the introduction of the relational database, in the early 1970's. The crux of a relational database is that data is arranged in tables with a unique key (ID) identifying each row.
One can install an RDMS, Relational Database Management System, on their local machine, or server. Next, create a table or tables at will. Then, run SQL, Structured Query Language, scripts to manipulate data. Then, eventually manage provisioning of said database to be accessed by other programs.
The service, Amazon RDS, exposes an RDMS that has been setup in distributed servers across the world which you access through your AWS portal, or otherwise, via the internet. The RDMS is managed for you, meaning that its underlying setup, operations and scaling is handled for you leaving you to do the high leverage actions to solving the problems you are working on.
Two major advantages for a software developer is its ease to use and relative cheap cost; the latter being due to economies of scale, and the open source nature of the DB used in Amazon RDS.