Roslyn is a project that extends the capabilities of well-established compiler tools by giving developers access to them directly in a service-like manner. What that means for .NET software engineers is that they are able to adjust the compilation process to their specific needs through an infrastructure that is provided to them by Microsoft. Specifically, low-level access to the compiler means that the developer can easily extend it and optimize some parts of the process by being able to directly interact with code that is being manipulated.
There is one video that I find interesting, that showcases how code analysis works with Roslyn and how the worflow itself is exposed externally to provide a potential integration hook outside the Microsoft Visual Studio IDE. Dustin Campbell, Senior Program Manager for Roslyn Compiler Services, is explaining the idea.
Here are some additional resources on Roslyn that you might want to check out: