PCB is "printed circuit board". it's the actual card itself, the bit with the pins on it that sits inside the plastic shell. Sometimes there is a code printed on the exposed part of the pcb above the pins, usually to mark what version the card is (like how there are different models of pcb for the gateway blue card, even though they all count as the same card)
It was just as suggestion and there might not actually be anything like that on the r4i gold card. But if cards are commonly faked then the makers of the original might add codes and checks to tell you if the card you bought is real or not. Acekard2i did this with the later revision of their card because people started selling fake acekard2i's.
Here's a look at the pcb's for a few different r4i cards (I don't think these are the 3ds version, but just an example). Something like the white text on the blue pcb is what I wasw talking about (it would be visible even wit the plastic shell on.
I believe this one is the pcb for an r4i gold 3ds rts
It's most likely legit, but just thought I'd give you a bit more info on pcb's and how you can identify them. If you buy that card then when you get it you should be able to see the small "C" in the bottom right when you flip it over and look at the back.