the silver shield thats given to youby your motherboard manufacturer(as each motherboard has a different layout) that is supposed to be installed in a rectagular shaped section of the back of the case.
heres an example:http://www.pecos-softwareworks.com/pics/foxconn_975x7ab-8ekrs2h/case_io_shield.jpg
the rectangular plate that is installed on the center left is the I/O shield for that motherboard. The one in the center is a detached version of a different motherboards I/O shield. As you can see, the slots arent the same, which is what i mean by different motherboards have different I/O shields.
On the same picture, at the bottom, you see a VGA and DVI port on the thin rectangular areas, thats the PCI backplate, and that's where the GPU will be.