"Caching" means that RealFlow | Cinema 4D writes the simulation data to disk and saves them for later use, e.g. for replaying the simulation or exchange with other programs.
Cache Folder (Mac OS X)
This is the OS X path to the folder where the simulation files will be stored.
Cache Folder (Windows)
This is the Windows path to the folder where the simulation files will be stored.
Use Cache
When enabled RealFlow | Cinema 4D will read existing cache files from the "Cache Folder (Mac OS X | Windows)".
Cache Simulation
A click on this button performs the simulation "offline". This means that you will see a progress bar, but the particles and/or meshes will not be visible in Cinema 4D's viewport. After the completion of the simulation "Use Cache" is enabled and you can play back the result.
Remove Cache
Be careful with this button, because a click on it will remove all simulation files and empties the cache folder. This action cannot be undone!