Saddle is a modular server software for Minecraft Bedrock Edition, built on top of dragonfly. It aims to make it easy both for developers to make plugins and for users to install and use them. For a comparison to dragonfly, please read here. If you are looking to run a saddle server with plugins, you might want to take a look at the launcher.
NOTE: Saddle is currently in it’s very early stages. It is currently just a concept, and everything from its internals to the name are subject to change. A lot of APIs are not yet implemented. Saddle is by no means ready to be used for a production server.
Saddle compared to Dragonfly
While saddle is built on top of dragonfly, using it as a library, it is pretty different from and not affiliated with dragonfly. They are both trying to achieve a different goal. While dragonfly is aimed at developers and more technical users, saddle cam be used by everyone looking to create their own server but may not know how to code.
However, even if you do know how to code you might still have a reason to use Saddle, as it provides an interface for different logic to work together. In dragonfly there are no such standards built in.