– A type of memory module that has a register between the DRAM modules and the system's memory controller. Buffered memory is also called registered memory, it is more stable than unbuffered memo [..]

This is when there is so much memory the chipset needs assistance to deal with the large loading introduced by the large amounts of memory. A buffer isolates the memory from the controller to minimize the load the chipset sees. See buffer and z-buffering.