It's simple: Groovy for Forge. With this, you can easily make Minecraft mods using Groovy instead of Java (or another JVM language). This mod library is required for any mods than use Groovy.
For developer information, see here.
Simply put this in your mod folder and any Groovy-based mods that use this library will load and stuff. There's really not much more to say about this, it's a library basically, doesn't add any new content or anything.
This is semi-experiment at the moment. It might not work. If you can get to work do show it off to me; for me my test mod compiled in Groovy failed the deobfuscation process.