Cache memory is a very small size memory, usually at 1MB or less, that resides in the CPU or is located very near it.  This memory is much faster than the standard RAM and holds data and instructions most often or recently requested in the system.  It works by anticipating which memory address the processor is going to call for, and loads that memory before the processor actually calls for it, then saving the data when the processor used it while discarding the oldest memory to accommodate the new one.

Since cache memory is mounted on-board, upgrading it would require upgrading your CPU or motherboard, or by buying a newer machine.