It's an in memory database that can be used for various tasks and is part of many open-source server applications.

The name helps understand a bit more what it is: REmote DIctionary Server. It is essentially a dictionary (key:value store) that runs on a server and can be accessed easily and quickly by multiple applications at the same time. Because of its speed and simplicity, it is used in many different ways in various applications. It also has a lot more features than a dictionary.