Its the Optical SPDIF Out Port.
Its an audio transfer format interface. It transfers digital audio signals from one device to another without the need to first convert to an analog signal, which can degrade audio quality.
You can connect a back panel S/PDIF bracket to the onboard header
You can also connect the onboard header to an S/PDIF input header available on some graphics cards using an S/PDIF cable. This connection routes digital audio to the HDMI output of the graphics card.