SPDIF (Sony/Philips Digital Interface) is a digital format, is used for transporting stereo digital audio signals on PC audio cards, CD players, DVD players, and other devices which transmit or receive stereo digital audio. If your motherboard has SPDIF header, you can connect such digital devices to your computer. There are two kinds of SPDIF connections: optical fiber and coaxial cable. Here is SPDIF pin definition for you reference.
If external graphics card support SPDIF in with two pins, please also make sure graphics card's pins definition. Then connect the two cables as,
1. SPDIF out from MB Pin#1 to SPDIF in to graphics.
2. Ground (GND) from MB Pin#4 to graphics Ground.