That lets you plug old-style "CardBus" cards (aka "PC Cards", aka "PCMCIA cards",) into a modern-style ExpressCard slot (such as is found on the 17" MacBook Pro, or the first couple generations of 15" MacBook Pro.)
Note: CardBus is based on PCI, ExpressCard is based on PCI Express (aka PCIe.) While PCI and PCIe are "instruction" compatible, they are electrically 100% different. Hence your original conundrum: You had a PCI slot, and a PCIe card. As you have discovered, one type of slot CAN be converted into the other (your $2 Goodwill card does it in the "mobile externally removable" space, while the card MacJunky linked to does it for desktop-size internal cards,) but it can be a pain, and it doesn't look good, in general.