Balm is a library mod for mod developers that simplifies the process of creating multi-loader mods by providing common interfaces and events and removing the need for most mod-loader specific code.
It is not a magic solution for running Forge and Fabric mods together. As a user, you only need to install this mod if you use a mod that requires it.
Key Points
How to make a mod with Balm (as a mod developer)
You can get started using this template repository. Documentation is limited, but the template gives an overview of most features and you can browse Blay's other mods' code to learn how specific things are done.