Ok so for anyone who has used Visual Studio 2013 Ultimate this isn't new at all, but with Visual Studio 2015 Microsoft has lowered the entry point and have made CodeLens available in the Professional SKU.
That's right, thanks to over three thousand votes on UserVoice, if your MSDN subscription, VSO Professional license or other method of upgrade entitles you to Visual Studio Professional 2015 (or Enterprise which has replaced Ultimate) you will quickly spot CodeLens appearing throughout your code, waiting quietly to serve. In my opinion it is one of the best features of Visual Studio and makes finding code changes and references a snap.
It's well worth reading up on as it can really improve your productivity and help cut some corners navigating large solutions.