That sets the maximum amount of system RAM that your video card is allowed to 'borrow' for use if it runs out of onboard memory. This has been discussed a zillion times here and in the Video forum; please search for way more information than you will ever need on the subject. It makes little performance difference either way; few games come close to using all of your onboard memory, even with "only" 128MB on the card.
I would leave it at the default unless you have a real good reason to change it. This is pretty good advice for all BIOS settings.