It's for Intel Turbo Memory. You need to actually have the mini pci-e flashcard in the slot; the BIOS setting simply permits detection. There's a driver for Vista and Windows 7.
If you have a Santa Rosa chipset (Latitude D series) it's pretty useless. If you have a Montevina chipset (Dell Latitude E series or newer), you can not only use larger cache modules, but have user pinning which allows you to choose which programs will remain in the cache.
I just did away with it and bought an SSD; this was meant to be a stopgap.