Everything exists in a game.
An example of a game would be the Sandbox game, or Pool.
The library project is our equivalent of nuget packages.
You can create a common library of code and publish it as a package. This enables you to re-use the same code over multiple games by adding the package as a reference.
Once you have made your library public you should be aware that if you remove or change the public API of the assembly you risk breaking projects that are referencing it.
In the future we'll be offering tools to help avoid this situation, like letting you reference a specific version of a package, and letting you test your library changes against other packages that are referencing it before publishing