Traditionally, compilers are black boxes – source code goes in one end and object files or assemblies come out the other end. The "Roslyn" project changes that model by opening up the Visual Basic and C# compilers as APIs.
To find out more, check out the following resources:
Note: This is a technology preview and there are known issues. While the shape of the public API is complete, only a subset of the Visual Basic and C# languages have been implemented at this time.