It is a code name for a new x64bit JIT compiler being developed by Microsoft.
A new, next-generation x64 JIT compiler that compiles code twice as fast is ready to change your impressions of 64-bit .NET code
The traditonal 64-bit JIT currently in .NET isn’t always fast to compile your code, meaning you have to rely on other technologies such as NGen or background JIT to achieve fast program startup.
This new JIT is twice as fast, meaning apps compiled with RyuJIT start up to 30% faster (Time spent in the JIT compiler is only one component of startup time, so the app doesn’t start twice as fast just because the JIT is twice as fast.) Moreover, the new JIT still produces great code that runs efficiently throughout the long run of a server process.