A Ramdisk is a disk drive that stores data in your computers memory (very fast) rather than on a physical disk drive (very slow). In a nutshell, you are sacrificing some of your computers memory in exchange for a very fast virtual disk drive. Second Life uses a disk cache to help speed up rezzing of repeat locations, placing this cache in a ram drive will dramatically boost it's performance.
This will not speed up the loading of locations the first time you visit them.
This is especially useful if your computer uses a solid state drive (SSD) as it will reduce the frequency of physical disk access and extend the life if your hardware.
This guide is written for Catznip running on Windows 8 64bit. Other viewers / Windows versions should work without issue but YMMV.