Sorry, I forgot how confusing that term first sounded when I first started getting into hardware
😛
The I/O shield, simply put, is the back of your computer where you would plug in USB cables, monitor cables to your GPU etc. it's the metal shield around them. So if you're looking at your computer when it's on it's side (horizontal). If the IO shield (cable plugin spots) are on your left, then the right side of the PCI slot should have this tab.
If it's a more modern motherboard, then it'll have a simpler tab. Either a hook or a switch kind of thing that looks like the ones on your RAM sticks.
PCI IO shield
PCI Lock tab: